Are there any horse riding trails in the area?

I was just there last week and saw quite a few.

There are some, yes.

In medora in the south unit there is an outfitter

Yes, most trails allow horses as Teddy Roosevelt was an avid rider. They are not allowed on the wind trail though.

While uncertain where horse riding trails might be, there are definitely places to ride horses. During my stay in the area, it was normal to see horse trailers, and even riders around Medora.