Electricity was discovered by the first guy looking at a thunderstorm, so they were called "thunder fish".
The word "electricity" was invented to describe the property of attracting straws after rubbing some wool on an "electron", which was associated with "sunbeam" and was the Greek word for amber (which in turn got its name after people conflated the ambergris or "gray amber" derived from sperm whales, with the fossilized tree resin or "yellow amber"... and the whales got their name from whale oil or "spermaceti"... which yes, people thought was whale semen that just happened to burn great in candles).
This, more that likely. Lightning has been around long before man poked out of his cave. And we know that back in the days of Vikings, pretty much anything that slithered was deemed a serpent.
They were discovered about a decade after Ben Franklin was credited with the kite experiment, not any record AFAIK what the natives called them before then, but the word Electric was commonly used to describe objects with electrostatic charge since at least a century earlier, making the Electric Eels name fittingly more akin to the shock they give than any actual unit of conductivity or charge they hold.
Actual serious answer: electric eels are exclusively South American, so the indigenous peoples there probably do have their own names for them which would likely pre-date the discovery of electricity.
(Electric fish known to other cultures would have been electric catfish, not electric eels.)
Follow-on serious answer: there are also electric rays, which are known as torpedos. According to Wikipedia, this is from the Latin torpidus meaning “paralyzed” or “numb” (the same root as the English “torpid”). The weapon is named after the fish. Edit: some of these live in the Mediterranean, and that Latin name predates understanding electricity; they were also known to Hippocrates who called them narke with a similar meaning in Greek.
IIRC some of the other pre-electricity names for electric fish are based on their ability to numb, paralyze or stun people and other creatures.