Animal Ghosts of Britain was a book by Mopsy Fleabert about animal spirits in Britain (Pm).
We know that there are animal ghosts because the Headless Hunt rides ghost horses. But we are told in OP38 that only witches and wizards can become ghosts; some sort of preparation or planning seems to be required. So, most likely, animals can become ghosts if a human performs the necessary magic on them. -BB