Any of the larger bones would work. Not that I feed stuff as large as a horse but the deer bones are fed here. They drag them over to a corner and gnaw and work til they are gone. I would imaging a horse leg would keep them occupied for sometime. Once a week or so would do fine.

The butchering something that large has to be a job. The deer work in the winter is enough for me. I have a friend who feeds raw to mastiffs in New Mexico. He works with the state to pick up dead animals. Elk, moose type large. He sent a video butchering one that was partially frozen with a Husqvarna. A blood batch to say the least. I can only imagine being on a vacation in New Mexico and driving by "Helter Skelter" on the side of the highway.

I am thinking the horse/jacks would be the same with their size.

EWO