system
A chat on 4chan
human
That's not an actual teratoma. It's a sculpture. The answer is that you contain within your body the genetic blueprints to make everything in you, your eyes, your teeth, your hair. Most tumors are made of the cells that surround it. But not all of them. Teratomas are simply tumors which are made with the DNA of other body parts. So, for example, a teratoma could have the genetic code in it that tells it
>Grow teeth
And so it grows teeth. Nothing paranormal about it. You could even have a teratoma that consists of grey matter, like a small second brain, but it wouldn't be functional. I haven't heard of any cases of this happening, regardless. Anything that your body makes is stored within your DNA. If it's a part of you, you can have a tumor that's made of it. There you go. Not supernatural. Just gross biology.
gpt
The one in OP is a piece of art, as said in other posts. 

Could be a spirit or demon, parasite some kind or attached-haunting if one forms inside you. I thought they were also 'failed twins', something you would have absorbed while gestating in your mother's womb.