I found the donkey while exploring the training centre place! Won't say where it is, you'll have to wander round the huge blank area on the map to find it!
I think I also found it by accident just now if its the one you both meaning ofc won't say where as that spoils the fun lol
i have never found those donkeys ever. no idea where it could be, and not about to waste time looking for it, even if someone does tell me exactly where it's hiding, i still probably won't find it.