I am trying to locate information about why one of my horses (a mini) might show his teeth at me. He wasn't being aggressive had just smiled at him and gave him a good scratching. I'm not sure if he enjoyed it or perhaps he smelled something on me and was reacting to it or what? I can't seem to find any information about this "smiling" and would appreciate any theories.

Hello,
When he showns his teeth, is he fleering (lifting his upper lip up, also called flehmen or horse smile)?

If this is the case, then he's doing it for two reasons:

Either he does smell something funny, or he's enjoying your itching.

When you scratch a horse, they naturally want to scratch you back-- which, of course, could be dangerous because of their teeth. Instead of actually scratching you back, they may just wiggle their lips or nip the air-- just like a dog will kick his hind legs when you rub his belly. It's sort of funny, but it's his way of saying "ooh, I like that!".

I've seen horses just nod their heads when being scratched, nip the air, stick their upper lips up, give a 'horse smile (flehmen), and even turn around and try to itch the person back! It's nothing to worry about, unless your mini tries to return the favor. His little teeth can hurt human skin, which is much thinner than that of a horse's-- so you just have to make sure he knows he can't itch you back.

If he does this mostly when you itch him, then it's probably because he's happy.

If he did this when you introduced him to a new smell or new medicine, then that's why-- horses often flehmen when they smell something new. It is thought that there is a sensory gland under the upper lip, and that this helps them remember the new scent, or perhaps identify it. Stallions will flehmen when they are sniffing the scent of a mare, to determine if she is in heat.