What are Thestrals? Well, if you only know Harry Potter from the movies and haven't read the books, you have seen these creatures already... kinda. Harry doesn't actually behold what Thestrals look like until Harry Potter and the Order of the Phoenix and Luna Lovegood assures him that he is not losing his mind. It takes a particular sort of person - someone who has had a certain kind of experience - to see these grotesque yet magnificent animals. They're probably one of my favorite magical creatures in the Harry Potter series. Expect to see them play a fairly big role in the upcoming movie adaptation of Order of the Phoenix.