Otherkin are people who believe that part of them is not human inside. They say that, inside their heads, they feel more like another animal than a human. That other animal might be real, like a wolf, or it might be fictional, like a dragon or an alien. Some otherkin say that they are vampires, by which they mean that they need to drink blood or take the energy away from people in order to stay healthy.
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. Basically, they are teenage saps who can't face reality and instead decide to spend their time on the internet, where they can bully other people who often "oppress" them. Any conversation or confrontation with otherkin is discouraged, as it will lead to blatant bashing and you will undeniably be proven wrong by their flawless logic, even though in some situations, they could use their inborn dragon, robot, alien or algae abilities to physically harm you.