Osmotic eels were an aquatic animal with medicinal uses.
In the 2150s, Doctor Phlox kept a female osmotic eel aboard the Enterprise. (Star Trek: Enterprise)
In 2151, Phlox used his eels to heal Captain Jonathan Archer's leg wound. (ENT episode: "Broken Bow")
In 2153, Ensign Hoshi Sato was responsible for feeding Phlox's eel. (ENT episode: "Exile")
In 2154, the eel was used to remove Xindi parasites from Sato. (ENT episode: "Countdown")
In 2155, Phlox used his eel to treat the wounded from the Tarod IX outpost after the Romulans attacked it. (ENT novel: Beneath the Raptor's Wing)