The electric eel, ;, is a sea monster in NetHack. It is feared for being one of the creatures to possess a a two-move instadeath drowning attack. It can also make your rings explode (in particular your worn ring of levitation...).
This applies especially to the Plane of Water. Many properly prepared players still have a somewhat irrational fear of them: a charged wand of teleportation (and MC3) will reliably avert their drowning attack, a single blessed potion of levitation will carry you safely through the Endgame, and the especially timid can have the entire Plane of Water to themselves if they blessed-genocide ";" right after entering.
While polymorphed into an electric eel, you can use the #sit extended command to lay an egg.
Eating an electric eel has a 47% chance of conveying shock resistance. Good luck reaching its corpse, however!