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

Nobody asked to be a cat! After dying in a fight against the forces of evil, a knight is shocked to discover that instead of being reborn as a hero of humanity, he starts his new life as a behemoth—one of the most powerful monsters in the world! The only problem is that a baby behemoth looks an awful lot like...an adorable house cat?! Things grow even more confusing when a beautiful elf adventurer rescues him from the brink of death rather than finishing him off. This house-cat-slash-knight’s baffling journey is just getting started...