Hrökkáll

The image is by "The book of creatures"

The Hrökkáll is a half-dead half-rotten creature that is said to live in Iceland.

Story
There was once a wizard, a wise one, he saw a dead half-rotten ell. So he revived the eel, but he didn't know that he gave rise to an evil and a toxic creature. When the wizard died the ell went to infest polluted waters.

Name meaning
The name means "Recoiled eel" in Icelandic, The cryptid was forgotten in Iceland and the name evolved to mean "electric eel".