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

After dying in a fight against the forces of evil, a knight found himself reincarnated as one of the most powerful monsters in the world: a behemoth! Problem is, he has to grow up before he can really strut his stuff, and a baby behemoth looks an awful lot like…a housecat?! And when an elf adventurer decides to take him in, she may need his help as much as he needs hers!