Electric eels were a type of fish which could generate electricity. They resembled eels.
In the 9th century, the Twelfth Doctor used electric eels to defeat the Mire. (TV: The Girl Who Died)
A giant electric eel lived in the watercourse below the Dalek City on Skaro which led to the Lake of Mutations. When the Monstrons attacked, the Dalek Emperor fell into the watercourse and was menaced by the giant eel. The Daleks captured the eel and utilised it for power. (COMIC: The Menace of the Monstrons)
Sleeks were giant electric eels which were time-sensitive. (AUDIO: The Thief Who Stole Time) On Funderell, the Wavewalkers worshipped a sleek known locally as Funderell's Daughter, or the Great Sleek. It served as their guardian. They consulted it for advice, and sacrificed to their god. Sartia killed the Great Sleek in self-defence with her staser. According to the Fourth Doctor, she committed deicide. (AUDIO: The Skin of the Sleek)