Are you looking for a horse trailer with living quarters, or a class C motorhome designed to carry horses (which would have to have very small living quarters)?
I have seen the former in many sizes, haven't paid attention to who makes them, just know there is more than one brand and they are sold by almost every horse trailer dealer in Oklahoma, Kansas, Texas. Two horses to eight horses, plus house, pulled by big pickups or small tractors, the tractor often a toterhome. Sized and priced for riders in rodeo, roundup, the lower end of racing. Big stables have bigger transporters.
Once beyond four stalls, the house no longer in the van, rather in the truck.
A C motorhome horse van, haven't seen it. Even a C toy hauler is rare, the platform isn't big enough for much space added to the house. I've seen big horse vans with living quarters in the UK, trailers and motorized. Typically the living quarters are for handlers and grooms, the owners of racing stables simply don't live in trucks or trailers. Racing in the UK is a big money business.