I only use fugly in terms of animals that are REALLY effing ugly. All horses are flawed, but a horse is only fugly if it's ridiculously awful. It's one of those words that's so strong, it doesn't quite fit as a term of endearment to me (same way I'd never use "tw*twaffle" as a term of endearment, but I might use "b!tch" when joking with a female friend.)