When they hatch, they quickly infect other local anemonefishes. Parasites, by definition, don’t aspire to kill their host. After all, if the host dies, the parasite loses its home and food source. However, in the case of the tongue-eating louse, some individuals can grow to be so large that the hapless anemonefish struggles to close its mouth. Eventually, as you might imagine, this hinders the host’s ability to eat.
A headless goddamned ghost appears right in goddamned front of them. You literally see it materialize on camera -- of all the entries on this list, this is the one you absolutely have to watch (here's a shorter version that cuts right to the ghost if you don't have 4 free minutes). Seriously, your mind will be blown like Eric Stoltz's penis in The Rules of Attraction. As they swing the camera lazily through the room, the ghost just walks very purposefully toward them like it's delivering a pizza, while a long, low moan emanates from its phantom lungs.