My Daughters Pony went nuts!?

My Daughter has 3 Miniature Horses and she rides them all, My hubby took the dogs for a walk down the paddock and Peppy one of the little geldings went nuts and attacked my brothers dog (who we are caring for) and was pawing at him and biting him. Normally Peppy does not care about the dogs as we have 3 of them. What would his problem be I don't want to let my daughter near him while he is acting crazy. Do you think maybe it is because he is a BIG dog or the fact that it was a strange dog. I don't know. Willie and Jazzy didn't seem to care about the Dog at all.