This is a creature from Myrm's Custom Creations: Draconis Glaucus mod

Level 190

In the wild, they are initially passive, lazily floating upside-down on the water’s surface near to groups of Cnidaria. To tame them, one must acquire an egg - first, feed the Glaucus a total of 10 Cnidaria corpses until it reaches max charge, which will cause it to attack you. Defeat it and it will drop an egg and flee.

Random Quality (low tier)

Random Quality (mid tier)

Random Quality (high tier)