Actually Tony did it himself and it wasn't pre-meditated.
Tony found out about the stable being burned and went to Ralph's house to tell him about it. Ralph, at this point, could care less because his son was dying in the hospital. Tony then accused him of killing the horse to get insurance money, which infuriated Ralph. "My kid is in the fuckin' hospital. And SO WHAT! It was a fuckin HORSE! An Animal!"
Tony then decks him and they begin brawling in Ralph's kitchen. Ralph tries to defend himself as best as he could, but Tony strangles him to death. He then calls Christopher to help him get rid of the body.
Dames