Actually, vampirism isn't fictional. Vampire bats live on the blood of other creatures. If enough of them flock to feed on, say, a small calf or lamb, they can easily kill it through blood loss. As for other creatures that seem to be monsters, just look at the natural world: there are amphibians that can regrow lost limbs, seahorse males that carry the young to term and give birth to them, bacteria that can eat your flesh away, mantis females that eat the head of the male WHILE he's mating with her, and frogs that can shoot blood out of their eyes, male fish of certain species that latch onto the flank of the much larger females, depositing sperm into her, while they waste away and are eventually dissolved into the flesh of the female, for just a few colorful examples. Sound like monsters to me.