I'm a Behemoth, an S-Ranked Monster, but Mistaken for a Cat, I Live as an Elf Girl's Pet, Vol. 1 (manga) — Nozomi Ginyoku

You've got to be kitten me!

A proud knight, slain in battle, finds himself alive once more-reincarnated in the body of a fearsome behemoth. As if he wasn't confused enough by this turn of events, apparently behemoth cubs look exactly like adorable kittens?! Even if the rest of the world sees him as soft and cuddly, he has the honor of a noble warrior and the strength of an unstoppable S-ranked monster, both of which will be put to the test as he fights to protect the beautiful elf who has taken him as her pet!