I'll give a little background for those who don't play video games. In most role-playing video games, you kill monsters for a lot of reasons. Oftentimes, these monsters yield "drops" ranging from common, uncommon, to rare. Most of the time, the said drops are parts of the monster's body.

For example, you kill 100 unicorns. In video games, probably the most common item you will get is some kind of monster meat (such as Unicorn Flesh x1 or Unicorn Hoof x1, or something. You get the idea). But at incredibly rare times, say, 1/100 Unicorns, you can get a Unicorn Horn x1.

Now my question is: Suppose all these monsters are identical, how would you justify these rare item drops? If unicorns all have horns why, out of 100 chances (just an example), you can only get 1 horn?

Also, please help me with the tags. Thanks!

P.S. I do not tolerate the killing of these sacred creatures

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    $\begingroup$ I remember playing a game a where creatures dropped imperfect items (broken horns or ruined eye) most of the time and only rarely did you get a perfect horn or intact eye. vendor trash does not always have to be something completely different. $\endgroup$ – John May 31 '18 at 2:58
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    $\begingroup$ I'm not really certain that this is appropriate for worldbuilding: Is it not basically asking how video game mechanics map onto in-universe phenomena? ...and on that note, as a kid playing video games, my kid brain came up with plenty good reasons for why that particular monster left that particular item. Then again, many people these days also complain that turn-based games don't make sense, either. $\endgroup$ – errantlinguist May 31 '18 at 3:18
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    $\begingroup$ @errantlinguist There are a lot of questions in this sub that borrows from video game mechanics. But regardless, isn't that still question for building a world? Regardless if it is used for a story, game, etc? $\endgroup$ – Bwrites May 31 '18 at 3:36
  • $\begingroup$ It's a game and they don't really require a reason for this. It's done to reward players for completing tasks and to encourage them to keep doing it. It's a player motivation mechanism and does not need a rational explanation. Collecting every item in a game is sometimes the goal of some players. You're overthinking this. $\endgroup$ – StephenG May 31 '18 at 8:46

I've played a very large variety of fantasy games from desktop RPG, to Rogue like, to modern games.

The basis for "drops" like you describe is that in combat your hacking and slashing the critter generally does not leave usable bits behind. Only rarely during a fight does the situation allow for an intact "Unicorn Horn" to be left by the defeated monster.

When you finally get that desired "drop" it is in effect saying that you slayed the critter and no part of the combat or the Unicorn harmed the valuable horn in any way and it is usable for whatever purpose that you want.

The other aspect to this is when randomizing loot (say for D&D), why do you roll on tables? The reality is each monster you fight during a dungeon crawl has survived this long by killing everything else it fought. You are seeing the drops of previous adventurers, other monster, and just random stuff the Orc found interesting.

A good DM (or game) should describe all the other "crap" (figurative and literal) the real treasure is mixed with. Quartz crystals might look very pretty to a goblin but are common enough to not be worth hauling out of the dungeon. Such a hoard probably contains broken equipment, rusty nails, bones, rotten hides, etc. The treasure roll is attempting to figure out whether anything killed by this monster had something out of the ordinary.

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    $\begingroup$ For tabletop RPG & if I was the DM, I'd tell the characters "OK but you're only going to do 25% damage" or some such. As for a video game, the makers would to build that mechanic into the game. You'd just need to figure out how to trigger that kind of combat. $\endgroup$ – Jim2B May 31 '18 at 2:45
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    $\begingroup$ @Bwrites who says those adventurers are the first to fight that creature? Maybe the last group to fight it and die damaged the valuable parts or maybe it is war broken by normal life, you will not find a a wild animals without some injuries they you have things like disease, mange, scars or even natural defects, look at deer antlers some are in horrible shape and antler size can vary drastically from individual to individual. $\endgroup$ – John May 31 '18 at 2:52
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    $\begingroup$ Regarding the "Unicorn Horn", I feel you should suggest to the OP to always include a "Unicorn Horn", just in various states. 10/100 "Broken Unicorn Horn", 89/100 "Grazed Unicorn Horn", 1/100 "Pristine Unicorn Horn". The horn is always there, but it's often not in the condition required by the quest. $\endgroup$ – Matthieu M. May 31 '18 at 6:53
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    $\begingroup$ @Bwrites: I like the idea of a careful fight; it could even be balanced, fighting carefully would lower the damage output (putting the character at risk since the fight is more difficult) in exchange of increase the drop rate of body parts in good conditions. You could even have several ways to do that: for example, if you want the skin in pristine condition, do not use a slashing or piercing weapon; if you want the bones, do not use a bludgeoning weapon, ... Just remember that you cannot fight at full efficiency and carefully. $\endgroup$ – Matthieu M. May 31 '18 at 6:56
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    $\begingroup$ @Bwrites: Learning how to kill enemies "carefully" sounds like a "get more drops from enemies" skill. $\endgroup$ – user2781 May 31 '18 at 7:39

You are looking for monsters with health conditions.

Suppose, for example, that a given magical sword can only be forged if you've got a liver that you extracted from someone's left side. Situs invertus is an actual real world condition that causes your internal organs to be mirrored. It affects 0.01% of the population, so if you are killing people at random, you might need to off 10,000 innocents before you find the liver you want.

For unicorns, notice how their horns are usually depicted as spiraled. Say that most unicorns have their horns spiraling clockwise... If you need a horn that spirals counterclockwise, and situs invertus is as rare in unicorns as it is in humans, then you have your justification for the need to kill 10,000 beasts before you find your drop.

Other conditions of interest: if you are going for newt eyes, you may need the eyes of those with aphakia. If a potion requires the feather of an angel's wing, it might actually mean a feather from the wing of a duck with angel wing disease. And so on.

Notice that many conditions may havbe no external signs, so you really need to open the creature to see whether it will give you the drop you want or not. Since this is usually lethal, and the creature will usually resist, you have the justification to make hunting the preferred method of farming1 for items.

1 For those unfamiliar, this is game jargon for such harvesting.

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  • $\begingroup$ If I'm only interested in counter-clockwise horns, why do I bother hunting the unicorns with clockwise horns? Surely I can tell from the distance... $\endgroup$ – John Dvorak May 31 '18 at 5:58
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    $\begingroup$ @JohnDvorak: The color variations may be too subtle to see from a great distance with the naked eye. Optics might not exist yet depending on the technology level. And the beast might become hostile as you approach. $\endgroup$ – Kevin May 31 '18 at 6:56
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    $\begingroup$ @Kevin ... or simply run away. Many animals have strong avoidance behaviour - they're not easy to observe up close. $\endgroup$ – Luaan May 31 '18 at 11:18
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    $\begingroup$ Even though i appreciate your answer's concept, this is definitely not how probability works. If i did my math properly (i did it fast, i admit), you actually need to kill 69K creatures to collect what you want with a 99.9% chance. :) $\endgroup$ – theGarz May 31 '18 at 11:59
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    $\begingroup$ @theGarz His intention was to provide an expected value. But, as he stated "you might need", he actually said that the probability at x=10k is non-zero (which is true for any x and any p between 0 and 1). Now, if you wanted the most likely unicorn ID to get your first clockwise horn from... it turns out to be x=1, because we're looking at an exponential distribution here. $\endgroup$ – John Dvorak May 31 '18 at 12:27

Perhaps they aren't actually identical? Say that 99 out of every 100 unicorns are unicornus simplicus, which as everyone knows has low-quality horns that won't hold magic worth a darn, whereas the 100th is unicornus incrediblus that is worth its weight in gold. But of course there are no outward signs of which is which (except perhaps to the trained eye), certainly none you could discern in the middle of combat.

(If anyone asks why the unicorns are outwardly identical - clearly it's an adaptation of the rare unicorn to blend in so it's not killed by marauding adventurers. Indeed, overharvesting by previous generations might be why they're rare.)

I don't know offhand any large real-world animal species that share ranges with related species to that extent, but it happens a lot with birds (note how detailed birdwatching guides have to be, compared with recognizing say a deer) and plants.

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  • $\begingroup$ Really like your answer! But if you can discern unicornus incrediblus from the rest, then they wouldn't really be rare anymore right? And the drop rate mechanic will no longer be justified? $\endgroup$ – Bwrites May 31 '18 at 1:15
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    $\begingroup$ Clever! Bayesian mimicry is a real thing. $\endgroup$ – Dubukay May 31 '18 at 1:19
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    $\begingroup$ @Bwrites Correct. You want to be unable to distinguish them at a glance. A post-mortem examination that can tell the difference is fine. Take for instance the monarch and viceroy butterflies: you can tell the difference on close examination, but discerning them in the wild is much trickier. $\endgroup$ – Cadence May 31 '18 at 1:27
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    $\begingroup$ They don't need to be different species of unicorns -- perhaps the horns themselves just naturally have varying quality. Something like hunting an animal for food and opening it up only to find that the meat is either too tough or (rarely) just absolutely fantastic quality. One could also draw some similarity with collecting geodes "in the wild" if the meat analogy is a little too gory. $\endgroup$ – A C May 31 '18 at 4:14
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    $\begingroup$ @Dubukay Do you mean Batesian mimicry? ;) $\endgroup$ – Korthalion May 31 '18 at 8:58

OK, let me focus on unicorns. I'll list only some possibilities and a creative person can surely invent more justifications. Note that you can apply different justifications for different types of creatures and if you think it's good for a game balance some of the reasons might be for example overcome by let's say getting more skills increasing the loot changes as your character develops.

Be creative and you can find other reasons as well for sure.

Note, some of those examples were already listed in previous answers, but I was always trying to add something from myself. Some are entirely genuine.

Not all horns are suitable

Think of a pearl. Not all shelled mollusk contain one and those that contain it usually have imperfect ones. Chances of finding a perfect one is likely less than 1:100. Similarly the unicorn horn might grow in various ways and only if it's given perfect conditions (e.g. no horn usage until the unicorn matures) the horn is properly shaped and can hold the magic. Depending on what you want to achieve from gaem balancing perspective you might have a possibility to distinguish "good" horns from "bad" before killing the creature or not (again - a skill that might be developed over time). Also you may have some sort of horn quality graduation (starting from those 96% totally useless, 3% average useful and 1% super high quality horns that are required for the most complex magic).

Another option is a rare mutation that causes the horn to have a different build. Again depending on the needs you either can't tell without killing the creature (the core is different) or you can (with increasing skills - the coverage is slightly different causing the horn to have slightly different shade of white, you need skilled eye and probably some magic to distinguish that from a distance or even closely - note you might not now if your horn is useful or not until you actually use it or find some other way to distinguish)

Horns get damaged during the battle

They can get damaged in a battle or it can get damaged over the time. Each option gives you additional opportunities.

If the horn can be damaged during the particular battle you might increase your chances by being more skilled and fighting in a less risky way (stealth kill?) - a skill that can be developed over time.

If the horn is damaged over the time you might learn to notice the difference before killing the unicorn. Again a skill to develop to be able to distinguish if the curse for killing the unicorn is worth it. In such case you might increase your chances by choosing younger unicorns. On the other hand the more mature creature is the more powerful the horn might be.

The horn gets damaged while looting

Horns are built of keratin that can be easily damaged while you're trying to detach the corn from the corpse. Of course you can always give it a try but you're lucky only 1 time in 100.

Again you can increase skills and that way increase your successful looting chances.

Stress causes horn damage

Of course it can be limited to magical abilities. You can't kill the beast not causing its stress however unicorns some might be more stress resistant and as a result the effect does not remove magical capabilities of a horn entirely. You can't tell from the distance and you can't tell to what stress was the unicorn already exposed.

Again a real life example. If you consider carp, this fish has a bad PR for it's intense "fish-like" taste. The truth is the flesh of carps get the taste when the fish are exposed to stress. The less stressful way of killing the fish, the less effect on the flesh taste.

Horns quickly loose their magical capabilities unless properly prepared

This time you need to act quick. But even creatures position might impact how quickly can you detach its horn. In this scenario killing from a distance actually reduces your chances of a successful loot. Skills can also be developed as well as you might need to have some other ingredients or mana power for a proper horn preservation.

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This is pretty much done from a game mechanic and balancing point of view. If Unicorn Horn is highly powerful and potent, then I simply can't afford to ruin the in-game balance and economy by making it easier to get, because that would over cause everyone to solely use this item and make the game boring or stagnant.

The way I justify it in my world is that you damage the part beyond use 99 times out of 100 and hence you won't pick it up because its junk. There are many ways you can justify this. Lets say you have to take the horn off the unicorn before it dies. Then I can have you only pick it up 1/100 times, because you weren't able to remove it before it died. You would of course have this all happen behind the scenes in a game, with the item dropping if you were considered successful. Or maybe the horn was chipped in battle and once its damaged, its power fades away. Game mechanics don't allow for this level of detail which you can include in world building or story telling because its too overly complicated for a game engine to determine this and not worth the investment.

If you wanted to, you can add in extra game mechanics to make it more real. Aka harvesting corpses for loot instead of just picking it up straight away. Most games just drop the items straight away and this has to do with it being a game and nothing to do with the world or real world logic. These things are meant to make the game more fun and easier for a player, rather than make everything a chore.

Here are some mechanics you often see in games which won't happen in real life but do in games

  • You can Run forever
  • Sleep as a mechanic to fast forward time rather than due to fatigue
  • Going to the bathroom or general cleaning
  • Being able to carry around an unreasonable amount of items
  • Fast travel
  • Generic conversation options
  • Eating/drinking as much food as you want instantly
  • Always being the chosen one or being special
  • Generic damage and criticals rather than body part specific damage
  • No bleed damage from cuts or stabs
  • Health Regeneration
  • Respawning
  • Instant crafting or crafting speeding up the more you do
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  • $\begingroup$ Clearly, my life is a video game because I have to go to the bathroom all the time $\endgroup$ – Dubukay May 31 '18 at 1:36
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    $\begingroup$ Health regeneration happens in real life too. It's more a matter of health not being expressable as a single number in real life. Damage rather than body part specific damage mirrors having a health number. Also, if you're a buddhist, respawning is not just fictive either. Always being the chosen one or being special is a matter of storytelling, the same applies to most protagonists in stories. Generic conversation options is effectively small talk. I can also vouch for sleeping as a mechanic to fast forward time rather than due to fatigue. Boredom sleeping is a thing. $\endgroup$ – Flater May 31 '18 at 6:14
  • $\begingroup$ @Flater I get what you mean, but have to admit games take it way to far. Take 20 bullets, wait 10 seconds. get shot in leg? wrap bandages around your arm.Need to wait 3 days for new shop stock, have 6 12 hour naps in a row, no problem. Wear a Bikini, more protection than full plate armor. Need more health? eat those 200 potatoes you've been saving since you were freed from prison. Missing something in life? install a mod onto life.exe and restart $\endgroup$ – Shadowzee May 31 '18 at 6:21
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    $\begingroup$ @Shadowzee: There are differences here. The bikini indeed makes no sense. Health regeneration, however, does makes sense, it's simply sped up to not bog down the experience; just like how stories can cut out boring travel time when not adding anything to the plot. Also, mods are different as well. What you're suggesting is akin to blaming JK Rowling for the bad Harry Potter fan fiction that has been written. $\endgroup$ – Flater May 31 '18 at 6:30
  • $\begingroup$ @Flater I've listed them as they are all mechanics akin to rare loot drop. Its a part of the mechanics and experience of a game. Sure I don't want to kill a 100 unicorns to get a horn, but doing so incentives me if it offers a good/rare item rather than saturating the game with everyone using and wearing exactly the same thing. No game is 100% realistic and you can see aspects of this in any game you play. $\endgroup$ – Shadowzee May 31 '18 at 6:43

Most drops are vendor trash

Many MMORPGs already use this mechanic. Every monster drops some unique junk item with no use other than to sell to the next merchant. Effectively this means the monster has a slightly higher gold value and serves to clog up your inventory.

Most unicorns drop a Scuffed unicorn horn. The horn has scratches on the surface from jousting with rivals or stripping bark off trees. Or it has grown slightly skewed. Or some interior imperfections make the material unsuitable for ornament or magical rituals. The scuffed horn can be sold to any vendor for 3cp who then sells it on to boil down and make glue.

One in a hundred unicorns drops an Immaculate unicorn horn. The horn is exatly straight with an unblemished surface, even spiralling, and no imperfections in the material. This horn can be used for jewelery or ritual or serve as a quest item.

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You, and most of the answers miss one thing: Video games are seldom about realism.

  • One Unicorn would give you enough meat for a year (if you had the means to conserve it). It´s unrealistic to drop only 1 meat and make 1 meal out of it.

  • Making the meal would require much more then two mouse clicks and 5 seconds processing time.

  • You´d have to leave most stuff behind, because you can´t cat carry that much. Certainly not in those pouches

  • You would not run around killing 100´s of unicorns. You would kill one, then cure your wounds and rest for some days. Then kill the next. Then the green party would show up an put you in jail for messing up the environment.

  • etc...

The reason with most of those points is: It´s more fun that way. The game mechanic has to reward the player enough to keep him motivated, but has to put up enough challenge to keep you occupied for some time. These are not worlds build for most accurate fantastic realism, but for maximum player satisfaction.

Fine-adjusting those aspects as drop-rates etc is called balancing the game, and is often done at the end, in a beta or even still after launch, after observing playtime, win-rate and other player statistics.

So, to answer your question:

Suppose all these monsters are identical, how would you justify these rare item drops?

Your justification lies outside the realm of this hypthetical world, in the form of maximum player satisfaction.

Note that in a well designed game, the Unicorn would always drop the horn, but be really really hard to find so you would never get 100 of them and be spammed with goblins insted

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