This is an elite Sea Monster of Lurker Below-style summoned by fishing.
It'll appears At Sri-La village in a pool named Large Swarm of Migrated Reef Octopus when the pool once being emptied. The Monster has 39,294k health. And his main attack is Gurrggggle, which just shoots a spout of water. No loots will dropped from it.
When the pool appears, an extra Angler “Salty El” will show up. And he’ll looks exactly same as you.