Otherkin are people who believe that part of them is not human inside. They say that, inside themselves, they feel more like another animal than a human. That other animal can be e.g. a wolf, a dragon or an alien. Some otherkin say that they are vampires and are usually acting like them, e.g. by drinking blood.
Otherkin is a neologism—a new word—and it is usually used only by people who say they are otherkin. The definition can change depending on who uses the word. Some otherkin say that their souls or minds are not human, that they were another species in a past life, or that being not-human is part of their religion. (Otherkin practice many different religions, however.) A few of them say that their physical bodies are not human, either, but there is no proof either for or against that.