Young foals don’t, but older fluffies will figure it out eventually. If they’re not taught what a mirror is, alicorns often take longer to figure it out because their instinct is to run away from the mirror.
For the most part, they don’t. A fluffy is a fluffy to them. Groups of strays band together for each other’s company, but they’re not strong enough to protect each other and they don’t have the foresight or the instinct to organize a rigid social structure unless they get the idea from an outside influence. Fluffies that acted more like the ferals in other canons used to be more common (for better or for worse), but most of them were wiped out due to both ecological concerns and their general unpleasantness. Most modern street fluffies are abandoned pets that can barely take care of themselves, let alone a herd or a litter of foals.
They can be trained and taught things, but the alicorn problem is a special case because the fear response to things that don’t quite look like a normal fluffy is hard-wired into them. I mentioned in another image I posted that the response that causes alicorn fear in my canon is an anti-bootlegging measure that can be tripped by all sorts of things, from alicorns to fluffy subspecies to mice. They won’t freak out and start trying to kill them, but they will run away from them.
I haven’t thought about that, but having the horn effectively be a giant penis on its head would be really funny. Maybe some have cartilaginous horns and others have fleshy ones considering all the different prototypes interbreeding in the early days. A fleshy horn might also start to droop if the fluffy has a health problem, which is both hilarious at the fluffy’s expense and has several different applications I can think of for a story. Someone might assume that their unicorn is healthy just because the horn stands up straight when that’s not a universal thing.