What does it mean when horses nip each other's necks?

If your Mini is nipping another horses neck and the other is spinning away or running away, the horse doing the nipping could be saying several things that all have the same outcome:

1. older mare is telling younger horses she is in charge
2. horses the same age trying to figure out who is the boss
3. horse doing the nipping wants some alone time or is annoyed with the other horse
4. Mare bitting stallion or kicking is telling him she's not ready to breed
5. Mare nipping another when she has a baby want's the other horse to stay away form her baby

Basically they want their space, having a bad day, or showing others who is above them in the pecking order.

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.