"This is the most complicated potion I've ever seen. Lacewing flies, leeches, fluxweed, and knotgrass."
--Hermione Granger, scanning polyjuice recipe (CS 10).
A leech is a small slug-like creature that lives in water. Leeches attach themselves to other creatures and suck their blood.
Leeches are an ingredient of Polyjuice Potion (CS10).
Leech juice is used in a Shrinking Solution (PA7).
The idea of leeches sucking out character traits, like they do in the Polyjuice Potion, originated in Europe in the Middle Ages, when they were used with the aim of removing excess or unwanted bodily fluids and balancing the humours.