They make peppermint horse treats so I can't see there being a health issue with it. I tried giving Vida an Altoids once...she spit it out too strong I guess
I think FoxyRoxy is right, everything in moderation is best. 1 mint candy or 2 isn't going to kill them but a couple of pounds might.
When we were kids we gave sugar cubes as a treat. Do they still make those?