I'm a Behemoth, an S-Ranked Monster, but Mistaken for a Cat, I Live as an Elf Girl's Pet, Vol. 8 (manga) — Nozomi Ginyoku

A Behemoth Meets an Elite Order!
After saving Gladstone and dispatching the belligerent monsters in the Underground Labyrinth, Aria and her friends have cemented themselves as heroes. The weary crew has no time for a catnap, though. Upon their return from the labyrinth, the party is surprised with a summons from the Emperor of Aushura himself! But his invite isn't for a simple reception -- it's a request to work with the empire and a band of legendary heroes to take down their most dangerous enemies yet: the Four Demon Beasts!