Terran leech

The Terran leech was a leech native to Earth. It consumed hemoglobin present in the blood of many other Earth organisms. 

Leeches were used as part of primitive medicine on Earth, and were often referred to when disparaging apparently outdated medical techniques. In 2374, the EMH Mark II aboard the USS Prometheus (Prometheus class) remarked to The Doctor that medical science had evolved beyond "scalpels or leeches" when the latter failed to recognize a thrombic modulator.