We have (pet) sheep and donkeys and they're perfectly happy together in the field, but when the sheep wandered into the donkeys paddock things took a nasty turn - the donkeys are clearly territorial about their paddock and it was lucky that the sheep weren't injured before we got them out, one of them narrowly avoided being kicked. I've never seen the donkeys so cross!
So that means we can't leave the gate open between the donkeys paddock and the field, which makes things complicated. If we want to let the donkeys wander between the field and their paddock/stable we have to make sure the sheep are in a different area.
The donkey sanctuary welfare officer told me that this is a very common situation.