The story
- Middle English · inherited fromhorseleche
- English todayhorse-leech
From Middle English horseleche, horse leche (“horse doctor; bloodsucker, leech”), equivalent to horse + leech. The worm is so called because it commonly attacks the membrane that lines the inside of the mouth and nostrils of animals, such as horses, that drink at pools where it lives.
Etymology from Wiktionary, CC BY-SA 4.0.