This big nasty seems to spawn in the final "hidden" zone of Shadowbringers and drops the horns needed for the coveted frog suit glam.

There's a chain of FATEs pertaining to it that involve helping an Ondo hunt it down, but after seeing said chain get completed three times in the course of a few hours, the boss FATE never showed.

It's possible it might just have an invisible timer like Foxy Lady, but the fact that I haven't seen a single reported spawn since launch makes me feel like there's some kind of other requirement that's being overlooked.

One of the level 80 dungeon quests does pertain to attempting to get information about it, so perhaps it simply can't spawn unless a player who's completed said quest participates in the last FATE?