I'm a Behemoth, an S-Ranked Monster, but Mistaken for a Cat, I Live as an Elf Girl's Pet, Vol. 2 (light novel) — Nozomi Ginyoku

He’s the cat’s meow! Tama is just getting used to his new life as a house-cat-slash-knight, adventuring with his master Aria and picking up new skills along the way. A brief visit to a nearby labyrinth, however, brings an unexpected turn of events: A wild, amnesiac girl lurks within its depths, and she wants just one thing—to mate with Tama?! To Tama’s bewilderment, Aria takes pity on the girl and adds her to the party, and it’s not long before Tama realizes why this crazed beauty seems so familiar…