The Animal Wife motif is common around the world and is unlikely to be solely related to the Otherworld. The standard version is an animal that turns into a human woman by shedding its skin. It does this in order to take part in a dance with its sisters but its skin is stolen by a watching human man who then lives with the animal-in-human-form as if married. At some point, the animal will rediscover its stolen skin, often with the aid of its unwitting, human children. The animal will leave, never to be seen again. It is rare for these animals-in-human-form to be men.
There is bound to be influence from the Otherworld in that the spirit forms make it possible for a human to take an animal wife or husband. However, the influence appears to be more in the other direction in that the Animal Wife tradition has shaped how stories about Otherworld creatures are told. It is unlikely that there have been as many human-creature offspring as there have been stories about animal wives. It is currently unknown (or unreported) whether Otherworld creatures can shift between human and animal forms once in our reality, although they cross in one form or the other, and they certainly do not need to remove or put on their skin in order to do so.
European traditions favour snake people and bear men as well as swan maids, whereas the most consistent animal wife stories in Blessed Isles traditions are Seal Folk, Fin Folk (by abduction) and Merrows (when they are not merely sirens). There are also animal husband stories in the form of the Black Bull of Norroway and The Brown Bear of Norway. Individuals in mythology and folklore have been turned into or discovered as various birds including swans, deer, otters, dogs and wolves but do not follow the full animal wife motif.