Eleking is a recurring eel-like monster in the Ultraman franchise.
History
Ultraseven
As a tadpole, Eleking was raised by two twin Alien Platt sisters. One was caught by a fisherman and two members of the Space Garrison, but not before she released Eleking into the water.
Eleking then began wreaking havoc. Since Dan was unable to transform into Ultraseven due to the Alien Pitt stealing Ultra Eye, he released one of his capsule monsters, Miclas, to fight Eleking while he recovered his Ultra Eye.
While Miclas failed to defeat Eleking, Dan was able to gain back his Ultra Eye and transformed into Ultraseven to engage Eleking in battle. Ultraseven triumphed, destroying Eleking with an Eye Slugger.
Ultraman Taro
Eleking appeared in the flashback in episode 25 as one of the members of Alien Empera's Empera Army.
Six years after its defeat by Ultraseven, Eleking rose from the ground on the night of a full moon, revived as Regenerated Eleking (or Re-Eleking for short). Re-Eleking rampaged through several villages, but was attacked and forced to retreat by ZAT.
Re-Eleking returned when another full moon rose. In response, Kotaro transformed into Ultraman Taro and fought the undead monster. During the fight, Taro ripped off Re-Eleking's horns, fatally wounded the monster.
Ultraman Story
Eleking was revived by Alien Mefilas as Re-Eleking once again. Taro was sent by Father of Ultra to defeat the eel-monster.
Juda later combined Eleking's spirit with the spirits of Alien Baltan, Red King, Gomora and Alien Hipporito to form Grand King.
Ultra Galaxy Mega Monster Battle
Eleking dragged Arston underwater shortly after it defeated Cherubim and shocked it to death. It later fought Gomora when Rei summoned it and beat Rei's monster down. However, ZAP SPACY used their Pendragon airship to lift Eleking onto land, allowing Gomora to defeat it. Rather than kill it, however, Rei instead captured him with his Battle Nizer, recruiting Eleking into his monster team. Eleking would go on to fight many monsters in the series.
Ultra Galaxy Mega Monster Battle: Never Ending Odyssey
Eleking meets his end in his fight against Tyrant. Though fighting bravely, Eleking was unable to overcome the monster and was killed by it.
Mega Monster Battle: Ultra Galaxy Legends The Movie
An Eleking appeared as a member of Ultraman Belial's 100 Monster Army. He fought against Ultraseven alongside several other monsters and was destroyed by Ultraseven's Emerium Ray.