Stanley

Stanley is a parasite/alien from a undiscovered far away planet. One day, his leader, a large ruthless creature, sent his parasites to different planets, to overcome and conquer each one. Stanley, was sent to Earth. The parasites can only transform into things they've seen, so the more things they see, the more things they can transform into. They can also just temporarily possess people and animals, but prefer to take over their body completely, which kills the host. Stanley first came to Earth, and took over the body of the first jogger he saw. He checked his wallet. "Stanley Williams." He's used that name and body ever since. The body doesn't decay while the parasite is in it, but if they leave the body for more than a day it completely rots. Stanley only ever leaves his human body when it's convenient, like if he's in the woods and sees a hunter, so he possess a deer for them to get close. Stanley can't die, but if you kill his host then he has no choice but to find a new one. He's taking over the world, one person at a time.