When we were in Badlands NP a couple years back, we came across a primative campground called Sage Creek. There were several people there that had horses. The reason I remember the ite is that lots of buffalo were also in the area, including 2 in the campground itself.

Yup, its in a NP, but i figured that horse campgrounds are far and few between.

That's one of my favorite remote campgrounds. When I was there, the only facilities for horses was the hitching posts, so it is largely do-it-yourself. Bring your own water and of course weed-free feed.
The bison can be aggressive* towards the horses so it is a good idea to bring some electric fencing for the horses.

*a cowboy type there told me "they will kill a horse in a heartbeat!".

Years later, I worked in a horse camp and they will indeed mess with the horses. This camp had six foot plus high metal corrals. One time a horse ended up in the adjacent corral, a bison had scared him so much!