Well, in Greek myth there is no hell, only the land of the dead; just one place where the spirits of the dead go. And besides them, I believe there is only Hades (discluding Cerebus and Charon, the ferryman, who are only met on the journey to the Underworld). In Celtic, again, I don't think there is a hell, just the Otherworld, or Underworld, where the spirits of the dead go. In both cases, it's only the length of the journey which differs depending on the good or evil of the person passing on. (The more horrible the person, the longer the journey is.)
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In Norse myth, there is the realm called Hel ruled by Loki's daughter, also named Hel. Like Loki's other offspring (Fenrir and Jormungandr), she is something of a monster, in her case, half-dead; she has the upper body of a normal woman, but her lower body is all rotting flesh. Like Greek and Celtic myth though, it appears to be the only land of the dead, and it is only called Hel after the ruler. Besides Hel herself, and the spirits that go there, I haven't found or heard of anything else living there. (There is Valhalla as well, but it is only where the dead heroes are found, to rise up and fight each other, sometimes feasting with Odin, before going back to being dead -- then rising up and fighting each other again, in a never ending cycle.)
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As far as I know, there is no name for a creature that started out as human, before becoming what it currently is. Just a Metamorph I guess (changing characteristics).