Sランクモンスターの《ベヒーモス》だけど、猫と間違われてエルフ娘の騎士（ペット）として暮らしてます•

Beheneko: The Elf-Girl's Cat is Secretly an S-Ranked Monster!

While fulfilling his sworn duty to defend humanity, a young knight meets a tragic end in a battle against demons. In a twist of fate, however, he finds himself in the lowest parts of a dungeon, ironically reincarnated as a Behemoth, an S-class monster of unparalleled strength and one of the very creatures he had once sworn to protect humans from. Beginning as a juvenile, he deceptively resembles an ordinary orange cat. Despite his seemingly overpowered ability to absorb the skills of any creature he devours, his escape from the dungeon leaves him gravely wounded. Just in time, Aria, a D-rank adventurer, stumbles upon him, decides to nurse him back to health, and ultimately takes him in as her pet, naming him Tama. Acknowledging Aria as his master and with his knight's honor from his past life on the line, Tama vows to defend her from danger at all costs—even if it means squandering this second chance at life. [Written by MAL Rewrite]