The horse-leech measures up to 15 centimetres. It is greenish in colour, and adults have a brownish-black back. Young horse-leeches have a lighter back with a distinctive symmetrical pattern. Some have yellow bands along the sides. It has a small attachment sucker, less than half its body width.
The horse-leech lives mostly around still water such as shallow ponds, or under stones near to water. It is common throughout Europe, Asia and the Far East.
The horse-leech swims by propelling itself forward in a vertical, wave-like pattern. Horse leeches are not dangerous to humans or large animals, as they are unable to bite through mammalian skin. They breed in summer, laying eggs in cocoons in moist sand at the edges of ponds or pools. The eggs take around 30 days to hatch and juveniles emerge completely formed. Horse-leeches are strong and voracious predators, feeding on worms (including other leeches), molluscs, water insect larvae and tadpoles. They are very sensitive to chemical traces in the water and find their prey this way.