I think there is an already-existing thread that explains it pretty simply. But I will take out a quote from that as it can be a bit lengthy for a new user to dig through it:
That's really all there is to it.
More details can be found in a blog I wrote which goes through it more extensively.
Yes, they are. As a matter of fact, I've been a creator and participator of quite a few of them myself. And further evidence? Well, there's the Character Database. When you create a character, there is an option that says "Non-Pony." You may think this is just for griffins and other such creatures, but it also applies to anything outside of MLP. Here ya go. A character I made that has NOTHING to do with ponay as further evidence in case you're super unsure.
Arylett Charnoa's post in Really Stupid Question - Roleplaying was marked as the answer