Electric eels were a type of fish which could generate electricity.
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)
On Funderell, the Wavewalkers worshipped a giant electric eel known locally as Funderell's Daughter, or the Great Sleek. They consulted it for advice, and sacrificed to their god. Romana II killed the Great Sleek in self-defence with Sartia's staser. According to the Fourth Doctor, she committed deucide. (AUDIO: The Skin of the Sleek)