Joulion is an enemy from the Metroid series.
A Joulion is a small, hovering entity resembling a bloated seahorse that appears in the Bottle Ship's Cryosphere area in the game, Metroid: Other M.
They will generally float about without attacking, but will occasionally generate a globe of purple-black energy. If Samus is near the Joulion, it will use that energy to project a bolt of electricity directly into the ground and charge around attempting to shock her, or when at extreme range they simply fire the globe at her.
They are vulnerable to all weapons, they inflate as they take damage, eventually exploding in a large burst of energy that will damage Samus if she is standing too close to them, or destroy other nearby Joulions.
The Joulion's ability to generate and manipulate intense electrical fields is similar to that of another Cryosphere bioweapon, the Kyratian.
It also highly resembles a type of water flea called Daphnia.
"Joulion" may have been derived from the term Joule, a common measure of energy, and Ion, an electrically-charged atom.
The Joulion's name may also have been derived from a Daphnia's Japanese name, which is mijinko.
Joulions are spawned as enemies on Pyrosphere in Super Smash Bros. for Wii U. They can be blown up by attacking them.
"These arthropods with hard exoskeletons are the larval form of Kihunters, a type of enemy from the Metroid series. In this game, they appear on the Pyrosphere stage, where they fall from above and then move about slowly. Throw them to attack your opponents or Ridley, or keep hold of one to make yourself harder to launch."