I've had a dog rim me, but I wasn't interested in the dog that way. It felt amazing.
My favorite sexual encounter was with 2 geldings. They buttholes were surprising clean, very dusty and dry, but clean. I recommend a clean warm damp rag to clean the area if need be and plenty of water near by to drink. If the horse is clean, their skin can be quite dry depending on a variety of conditions. So stay hydrated and expect them to fart in your face. The 2 I rimmed sure did. I got lucky their gas wasn't stinky if you pulled your head away, but when they let loose at point blank range it had quite a punch. I also stuck my tongue inside when they farted because I like to live on the edge and yeah I got some shit in my mouth. It wasn't great, but when your that horny it just kind of feels like a toll to pay to have a good time. Occasionally I would stick my head out of their butt and spit out the nastiness, then get back too it. I did that for over an hour, pure heaven.