Lampreys are ancient, elongated fish that look more like aliens than vampires (or fish, for that matter). They have no jaws, no scales, and spend most of their lives as harmless larvae. It can take up to seven years for one to reach adulthood, but once it does, it becomes a monster: Adult lampreys latch onto a host with their hook-like teeth and gulp down its blood as it swims, unable to defend itself.
Lampreys live in fresh and salty water worldwide, but while they already terrorize their own habitats, they can be even worse as an invasive species. When manmade canals let Atlantic sea lampreys invade the Great Lakes in the 1800s, they outcompeted smaller lake lampreys and decimated native fish, some of which are now extinct. They only attack humans when starved, however — a rare problem for such successful hunters.
Check out the video below for an up-close look at captive sea lampreys: