Board Thread:General Discussion/@comment-25850402-20150723062747/@comment-25926288-20150723150939

Poseidon isn't a merperson. He is a deity. There's no proof their is  a difference between divine magic and witchcraft. Goddess Ursula, Poseidon and Regina can all change mertails into legs, so there's no shown difference between that and witchcraft magic.

Poseidon can just be an immortal person with magic. Merlin is shown to live for thousands of years and have magic, but he isn't a deity. Same with the Dark One.