It’s usually sheep or goats in this type of joke, probably because they’re fluffy, small and everything is, well, easily accessible. Pigs might also work anatomically but they’re mostly associated with being dirty (which they’re not really but we’re not talking about high-brow entertainment here). Cows and horses are too tall and also can seriously hurt you.

… I am not advocating in favour of raping sheep or goats. I’m just explicating why they seem to be “obvious” victims. Also, in reality, people have stuck their penises into any available vagina.