Bull penis is long & skinny. I've seen a video of a guy deep throating 1 but I don't know how much length he got. A bull sticks his cock out, cums & he's done so I wouldn't imagine it's physically pleasant for all the trouble you'd go through
The thing is when I say they're long I mean they can be up to 2 1/2 feet long
A Clydesdale would require extreme stretching 1st & even then the flare would hurt like a motherfucker, that's assuming the stallion doesn't suddenly thrust & rip her apart... try explaining that in the ER while she's in shock & bleeding out
These belong in fantasy not reality, sorry.