It's actually the cannon bone above the ankle, but I believe his ankle is fused for support. When you look close you can see where the injury occurred; a displaced condylar fracture just above the fetlock joint. It resembles Saratoga Six's (may he rest in peace), but actually a lot better looking.

Jump Start is literally the kindest stallion I've ever been around, and one of the largest. He'll get you if you're not paying attention simply because he can, but you can play with him without ever worrying that he'll lay his ears back and bite you from pure spite. I'm happy Jump Start is doing so well, really.