A leech is a mutated insect found throughout the waterways in Hoofington or in damp environments, like swamps. They are the primary source of food for Hoppers. Their meat is also edible, as shown by Rampage who was cooking one of them. The leeches are much bigger than their unmutated ancestors. They will latch onto passing boats and can be made to loosen their grip with a simple poke.
The leeches were first encountered by Blackjack as she made her way downriver, in the Seahorse. They were encountered again, living in a swamp and several smaller ones had latched onto Blackjack by the time she, Scotch Tape and P-21 made it out of the swamp.
The leeches suck on blood as their main source of food. It is likely that they feed on other river predators, like Hoppers and Radigators. They require wet or damp environments to survive, limiting them to certain locales like swamps and rivers.