Kicking with its hind legs and hooves. The hooves can kill an animal and seriously hurt a human. Either that, or bite with its strong, wide teeth. Also as dangerous. Don't underestimate ponies XD
A horse primarily escapes danger by fleeing. However, when cornered, the animal can lash out physically, relying on bucking, biting, kicking, rearing and striking to drive predators away. A horse generally prefers to be kept somewhere open where it can run away if necessary.