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.) m 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.) m 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).
In Hindu and Buddhist mythologies there are many hells, some of them radically different from others. The more fiery ones, similarly to the abyss called Tartaros in Greek mythology, are inhabited by different brands of sinners suffering for millions upon millions of years until they make up for their misdeeds in life. There are several hells which are, however, more like underground worlds, or like the "Underworlds" of Western mythologies, conceived of as existing under the earth where the living reside. There are many different races of creatures whose names are most often translated into English as "demons." These include some tribes of a race of snake-people called the Nāgas, as well as several members of the various races of āsuras (beings considered to be the traditional enemies of the gods) called the Daityas and the Danavas. These last two are groups of gigantic āsuras whose first members were actually the cousins and half-siblings of the gods, much like the Titans in Greek mythology. Some hells are inhabited by many other sorts of more demonic āsuras, such as the goblin-like Piśācas, the often gigantic ogre-like Rākṣasas, the elemental protective spirits called the Bhutas, and the more well-known spirits or ogres called the Yakṣas. From the Danavas and Daityas were descended a number of notorious tribes or nations of āsuras, such as the Saiṃhikeyas, the Paulomas, the Kalakeyas and the Nivatakavacas, a lot of whom lived in the aforementioned underworlds.
I'm not aware of any specific word describing a formerly human creature apart from the name of whatever creature s\he eventually became, e.g., vampire, werewolf, ghost, monster, god, nymph, etc.