How would this have been weaponised
There are lots of different kinds of hair, and a plethora of hair like objects.
Normal human hair: Not as interesting but it has many potential uses
- Rope. Made famous by Rapunzel of course
- Sling. Small patch of leather around a long braid and bam. Goliath is down
- Garrote. Strangling people is a great way to be sneaky
- Disguise. Grow it, dye it, use it to obscure features
Other mammalian hair: Have you ever looked at elephant hair or bear hair?
- Defense. Maybe enemies throw beehives...who knows. If you could grow the super thick hair of an elephant it would be a great way to keep insects at bay if you can make it thick enough
- Survival. While this could be done with human hair as well there are better more specialized versions. Sea Otter fur for example is so dense and thick that even though they live almost completely in the water their skin doesn't get wet. This variety could help soldiers survive in cold and wet climates much more easily.
Quills and spines: These are awesome and an amazing method of melee defense and nothing more than specialized hair. Quills generally require a specialized sub-dermal muscle. This both holds them in place and allows them to move. Its not practical to have them sticking out all the time so it has to be possible to lay them down
- Defense. Obviously for fist fights there isn't a much better natural defense
- Small piercing weapons. Otherwise known as a dagger. This would be a great skill for an assassin to have. Metal detectors are now virtually useless.
- Projectiles. Its always fun to consider them shooting out ala Hydralisks of StarCraft fame...that would require a whole other conversation about propulsion systems.
Horns, talons and claws and antlers...so many options! If you want to stick with hair that's fine but its not a distant jump from hair to specialized finger nails so its worth some consideration.
Natural Weapons. Pretty obvious. Talons and claws are sharp, they are after all designed to grip/pierce flesh. A couple considerations: Can your humanoids simply grow them? Can they also absorb them back into themselves after use or...what. Talons are great for a fight but uh...that bathroom thing would be a tad dangerous if you had six inch needle sharp talons.
Impromptu antler weapons. Antlers vary in a significant way from horns. While horns are a permanent and ever growing features for those animals with them, antlers fall off every year. If you could replicate the growing process over a short time frame it could fall off and bam, insta-weapon. Better yet if you can control the shape of the growth you could make any range of moderately effective weapons, you could even make an antler boomerang!
Horns...how awesome are horns...sorry, I was just imagining being able to head butt a co-worker. Sorry. But actually that illustrates my point pretty well. Horns exist both to attract mates and offer protection when the super intelligent males smash their heads together to impress the ladies. Its a good thing humans don't do that...
Would it have caused a difference in how weapons were developed?
Undoubtedly it would have, at least for a time. These options are all still quite primitive compared to anything beyond stone age weaponry. Once you introduce metal into arms and armor the growth abilities become severely limited in their usefulness...I can see them making bar fights more interesting...but all in all hair and horns just can't compete with bronze, iron, steel and firearms in an appreciable way.
There is one refuge I can think of for the ability into essentially any era though and that is the spec ops/assassin world.
Being able to grow a dagger or razor sharp claws means never having to worry about a metal detector.