what is this on horses leg??

I've seen them get as long as your hand on some drafts. They get trimmed every time their hooves do. Easy enough to do if you can see the entire area. Ours get irritated with you if you try to twist them off so trimming them it is.

Yes, our quarters and paints have them too. Just much smaller and have never needed to be trimmed or pulled off.

My Felco's get put into service for that too which my landscaper friends find hysterically funny that I "prune" my horses.