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In many RPG games, there exists a (usually evil) creature whose body mass primarily consists of a giant eye, and is usually at least the size of a human head.
Sometimes it has wings and flies, sometimes it has tentacles and floats through the air. Most of the time, the eye's gaze is dangerous in some way — whether it shoots lasers or otherwise incapacitates a target.
Is there a realistic way such creatures could evolve? How close could we get within the realm of realism?
Ooh... Interesting problem. I'm going to focus on a potential evolution path rather than just saying 'it's unlikely' because it's more fun!
The biggest issues are where to put the organs and how to make it float. If we look at traditional images of beholders floating eye creatures we can see that they're depicted as having a mouth, so we can assume they eat and don't photosynthesize etc. The sharp teeth also suggests carnivore, which helps with the energy requirements somewhat. The space given over to the mouth also gives an opportunity for a compact in-situ digestive system, somewhat similar to a snake crossed with a pelican. Once prey is incapacitated (which I'll get to) the beholder floating eye creature reticulates its jaw, expands its 'digestive pouch' (gullet merged into stomach), swallows the prey like a snake and sits, grounded, while it digests it. When it's done it spits out the bones and flies off. Our beholder floating eye creature has a vestigial anus, just as a point of interest.
Obviously the above suggests an evolutionary track from a snake, or an ancestor of snakes, so we're likely looking at a cold blooded creature. From here we get a possible evolutionary track for flight, as creatures that can rise above low lying fog etc would be able to stay active longer than ones that can't, matching up neatly with the hunting patterns of the beholder floating eye creature (why hunt prey when you can eat it as it sleeps?). There are a number of gases that could help with the floating, some of which may be byproducts from the digestive fluids used. This creature is grounded while eating, and eats to avoid being grounded. How paradoxical.
The other organs can be distributed neatly around the flesh of the creature. There's a lot of surface area to fill with (very flattened but still functional) organs, and if you're happy moving some functions out into the tentacles (a-la octopus) then things like liver/kidneys etc can be shifted fairly easily. The brain is a little trickier, as it needs to be fairly localized, but this gives us a wonderful reason for why they have such big eyes in the first place.
Flies (and many insects) do a lot of processing in specialized neural clusters fastened directly to the eyes. This has a lot of advantages for them, as it lets them react faster as the distance from input-reflex is much shorter. Now imagine a lizard living in low lying fog. It might have only a millisecond before it can see a predator dropping down from above, so it needs to be able to see, think and react very quickly to survive. As such: It develops specialized processing that links to it's defensive system (again, I'll get to it in a second) and is situated directly behind the eyeball (I can't explain the mono-eye, we've got to go very far back in the evolutionary chain for that). Over time this eye-brain develops, and the creatures head adapts commensurately. The musculature for ocular motion is a tricky one, but it's possible if you assume the creature hasn't got a massive amount of 'scan' to the eyeball (it can't track massively far, preferring instead to turn it's head) The larger surface area allows the creature to utilize some of the previously wasted gases for lift, and it starts to use a combination of jumping and tentacles to 'fly'. The body and legs become vestigial, and you have your beholder floating eye creature. One major concern is the weight of the eye, but it's not inconceivable to have an eye held up by struts instead of fluid (though it is weird. Perhaps a part of the 'single eye' evolution way back in the dawn of time?)
Now the defensive system becomes a hunting system for immobilizing prey. What is the defensive system? A series of tentacles with stings on the end. In the early lizard the eyes were used to rapidly orient the stings towards incoming predators, reducing the amount of energy needing to be expended on growing all-round spiny coverage. In the later lizards where the eye is taking more direct control of signal processing the tentacles become a predation system as well as a tool for manipulation and flight (flight might need some specialized 'paddle like' tentacles)
On the subject of using the eye as a weapon: I can't think of any way to make it directly offensive, but if the eye is photo-luminescent it would act as a lure for any stray creatures lost in the fog, enabling a grounded beholder floating eye creature to lure, sting and devour a creature to refill its floats. I think that would count as dangerous.
TL:DR : It evolved from a one eyed lizard in a very foggy place. This explains its fondness for crypts, dungeons, and mysterious mists.
Addendum: On the eye structure: A fish-like creature in the dim and distant past was predated upon by smaller fish that would nip at its soft fleshy eyeballs. Over time those eyeballs grew hardened and chitinous, leading to diminished visual acuity but stronger eyes. As these eyes formed they developed internal strutting to hold the eyeballs rigid rather than risking cracking/stress injuries. When this fish's ancestors eventually made it to land their eyes drained of most fluid, leaving them with a hollow (light) eyeball and terrible eyesight. This explains the lizard's weird requirement for faster eye-reaction time (it's almost blind), it's predilection for foggy places (Why live where you've got the shortest vision? Go somewhere everyone is blind!) and (almost as an afterthought) The unique angular patterns some beholders floating eye creatures seem to have in their eyes.
Usually these things are depicted much like a human eye, i.e. with a lens in front — which means that the body needs to be hollow, which leaves no place for muscles, a digestive system or a brain that could do anything with the sensory input. So this is not very likely, especially if you want it to fly.
If you are prepared to settle for a spherical thing that responds to light I could imagine a Volvox-like colony of light-sensitive cells that respond to changes in brightness — they could "see" people in as far as people block out some light. This colony might even evolve in a way that most cells are opaque and only a "pupil" is transparent — this would allow the colony to better orient itself towards the light (move until the transparent bit is fully lit) and might give the impression of an human eye.
Volvox is aquatic, but maybe our hypothetical cells produce a gas that is lighter than air and lets them float through the air in the way of a helium balloon.
I cannot imagine a mechanism that lets organisms shoot laser beams, but there might be nematocysts placed around the "pupil" that sting in the way of jellyfish and thus incapacitate prey or attackers.
Updated: okay, there seems to be an all-eyeball creature (although it's not gigantic and I doubt it would scale well): Single cell eyeball creature startles scientists
Basically, no, not literally. They don't physically make any real sense, as animals. In fact, the point seems to largely be that they don't make sense. Almost everything about them is the opposite of making sense. What makes sense about them is that they're terrifying, but they're the kind of terrifying that is scary because it defies reality and makes no sense, so seeing it in reality would tend to be extremely alarming, not just because it's icky but because it violates our understanding of how things are supposed to work (and NOT work)! A central theme of horror is the unknown, the unexpected, the unknowable, the incomprehensible, the unimaginable, the unfathomable, and things that seem to contradict what we think we know about our world.
So... maybe the reality is different from the perception. Such a thing can exist if it is a nightmare, an illusion, magic, something else disguised to look like something that shouldn't be able to exist, and/or a horror story that people believe in and are too scared to go anywhere near it to verify it. As a conjured horror created intentionally, it makes sense if it can be engineered or magicked or conjured or whatever, but not as a naturally evolving animal.
It is possible that a creature with massive eyes could eventually drop the need to eat. As a result, it could eventually move past most of the rest of its form. It's also possible that the eye could be designed to eject from its body and eventually such an eye could start to reproduce and/or survive on its own. In real life, some creatures like frogs can shoot poison or blood from their eyes. This could be adapted to be even more dangerous. You could also intensify the crystallin levels in the eye that form the lens and use it to concentrate a laser... With a little magic in there somewhere.
Does the housefly count? Most insects have compound eyes along with simple eyes and if you calculate the ratio of eye size versus total body size, you would find that their eyes are the largest in all animal kingdom.
Or Ophthalmosaurus (a type of ichthyosaur)? http://ncbi.nlm.nih.gov/pmc/articles/PMC1771016 states that (in vertebrates) this animal had the largest eye-to-aggregate size ratio.
Considering that I am editing my answer to include details and reference links, it is also appropriate to answer a logical question in the comments here:
@YoustayIgo Is there anything which prevents eyes that size to go along with an even smaller body? Just because it hasn't been done that we know of doesn't mean it can't. – DoubleDouble
There are two sides of this question. One scientific and the other logical. I would answer the scientific side first. As far as I know there has been no detailed research on the factors which limit eye size. If critic finds that a larger eye size is possible with a creature smaller than Ophthalmosaurus (19 feet) then he/she should present that research for the benefit of all members. Even if the said research does not exist, the critic can still research the need for larger eyes, the resource requirements and the risk factors involved and post them here. It would not count as a scientific research, but it would be still better than simply raising a meaningless question without any scientific backing.
As for the logical side, the answer is simple. No, as far as we know, there are no logical constraints for such a creature. But that does not mean that such a creature does positively exist. For example, one could theorize that logically it is possible that one group of mammals quickly evolved into paleo-humans during the time of dinosaurs and lived alongside. They used to ride on suropods and hunted mega-theropods like T.Rex for sport. Unfortunately, one day they encountered a cookie-monster who killed them all and erased all evidence of their existence so that we don't find any evidence of their existence. While it is theoretically possible that the above absurd tale is true, no sane person would give it a second thought. The same applies for a giant-eyed creature, smaller than 19 feet.
Logical possibilities about organisms' design are not worth consideration unless there is dedicated scientific research backing them or the discovery of any evidence for the presence of such creatures.
Here is an image of Ophthalmosaurus.
