https://www.gamespot.com/articles/final-fantasy-14-adds-monster-hunters-rathalos-in-/1100-6460992/

Last week saw Final Fantasy's Behemoth cross over into Monster Hunter World, and now a monster from Capcom's series has invaded the world of Eorzea. The fearsome Rathalos arrives as part of Final Fantasy 14's big 4.36 patch, which introduces an assortment of Monster Hunter-inspired content, along with a new area to explore in the Forbidden Land of Eureka, to the MMO.

Rathalos is the target of "The New King on the Block," a quest available to players who have reached level 70 and completed Stormblood's main campaign. If you meet those prerequisites, you can accept the mission by speaking to the hearty hunter found in Kugane.