S Rank Monster no (Behemoth) dakedo, Neko to Machigawarete Elf Musume no Kishi Pet to Shite Kurashitemasu

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...

[From Amazon]