Top: Bichir and trunkfish [top], Electric Catfish [bottom]
Center: Electric “eel” - Electrophorus electricus
Bottom: Indo-Pacific Moray Eel - Muraena nudivomer (now Gymnothorax nudivomer) 

A while ago I saw this Bird and Moon illustration of animals with misleading names, but I kept seeing people asking, “Ok, if they’re not THAT, then what ARE they?” For some reason, I completely forgot that I wanted to cover those questions, but hey, better late than never!

The electric eel isn’t an eel - it’s a knifefish. Knifefish (Gymnotiformes) are actually more closely related to electric catfish (Siluriformes) than they are to true eels (order Anguilliformes), but developed their electroconductive organs through convergent evolution - the first signs of the organ evolution in both the electric eel and the electric catfish appeared after they shared a common ancestor.

In addition to electric eels and electric catfish, electric rays (order Torpediniformes) are the only other “strongly electric” fishes - that is, fish that produce electric shocks over one volt, and use their electrogenerative organs to either stun or kill prey and/or attackers. There are many fish that can produce a small current (“weakly electric”), but it is used for electrolocation and electrocommunication, instead.

Images:
Fishes of Zanzibar: Acanthopterygii. J. Van Voorst, 1866.
The Standard Natural History. John Sterling Kingsley, 1884.
Wild life of the world. Richard Lydekker, 1915.