For some of them, it's due to spiritual beliefs involving reincarnation. For those individuals, they have an awakening involving recovered memories of being an animal or some other type of creature in a past life. If it's not a creature from Earth, then there might be past life beliefs combined with parallel universe beliefs or that their soul is from another planet. They know physically they are human, but they believe their soul is at least partly animal or some other creature. For others, it might be due to being an animal/creature DID alter. In that case, they feel that they are not human, but have to exist in a human body.
Regardless of why, which can differ between individuals, from time to time, these nonhuman identifying individuals have mental shifts that cause animalistic urges and phantom limb like feelings such as feeling like they have wings or a tail.
Some of them express their identity by making gear related to what they feel like they are (masks, ears, tails, paws, wings, etc.) and by taking part in activities related to what they are. Some activities might be running on all fours, going on walks in nature, playing with pet toys, digging in the dirt with their hands/paws, curling up on some blanket in an animal way (symbolically a nest or pet bed), or making animal noises.
The shifts/urges can happen whenever, so sometimes it's not a good idea to give into those urges at that moment because they might have to be responsible “people” or because they’re not in an environment that will be accepting of them. For some, fighting against the urges can be tough and cause a lot of stress mentally. There are also a few that go through species dysphoria due to the phantom limbs or because of not having the limbs they feel like they should have (wings, tails, animal ears, etc) and having a deep feeling that they should have those things. Because of that, they feel uncomfortable with their human body and can’t do much of anything to fix that (other than gear).