Ok i had this problem, but not to this extent.
I have 2 horses, and 2 ponies. We got the two ponies on April 9th, and they lived in the pasture for two days by themselves. Other than being stressed from moving out of the barn Phillip (the aggressive pony) lived in for 7 years, he was fine. He was purchased with Savannah, so we kinda got a 2-in-1 deal, she came to keep him calm. On April 11th, Storm and Zeph came (ALL the way from Mississippi) the horses were fine, and just curious about their new pasture mates. Now, Savannah is the only mare, and guess who thinks hes her "mate. Yup, Phillip. Now when i say these two are ponies, i mean it. Savannah it 12.2, and Phillip is 13.2. The big horses are 15.1 (Storm) and 15.3 (Zeph). Now Zeph it a Right Brian Extrovert, so hes oh-so-curious. He went to see the new ponies, and Phillip canters up and tries to bite him, then tries to with Storm. It runs the big horses off, but not for long. Zeph comes to sniff Savannah (who happens to be in season) and Phillip goes into a bucking fest. Then Savannah sticks her tail straight up in the air and pranches circles around the two big geldings, like she teasing them. Later in the week, Savannah is hanging around Zeph, and Phillip noices, he GALLOPS up and takes a chunk out of Zephs butt, and as he bites Zeph, Phillip gets a blow to te neck. Put him in his place.
I suggest you get a totally new animal with this horse, like a couple of goats. xD