Hex is a crab animatronic with a completely different shape than any other animatronic. If you don't look at him constantly, he will charge at you with no sign that he is about to do it. He can move to another room and keep changing rooms, but if you don't find him and check on him again and again, he will charge you and kill you. Hex first appears at night 4.
Hex's jumpscare is that he might lean in one of the doors or jump right through your window in front of you, shattering it into tons of little pieces.