- older mare is telling younger horses she is in charge
- horses the same age trying to figure out who is the boss
- horse doing the nipping wants some alone time or is annoyed with the other horse
- Mare bitting stallion or kicking is telling him she's not ready to breed
- Mare nipping another when she has a baby want's the other horse to stay away form her baby
If they are nipping at other horses while your there, driving the other horse off, they are telling everyone that you are their friend and you are here to see them.