My horse bites my dog help!?

My horse keeps trying to bite my dog he actually bends down and tries to bite her she is tiny and she loves going to see him so I won't stop taking her. My horse was abuse really badly and if you even raise a hand to him he'll try to attack you so I can't really even tap him on the nose ! So I have no idea what to do my horse could break her ribs if he bites her cos she's really small what shall i do?