S-Rank Monster no Behemoth Dakedo, Neko to Machigawarete Erufu Musume no Kishi (Pet) Toshite Kurashitemasu

Elven girl, "Aria" is an adventurer. A small beast with an adorable face embraces Aria's fruitful chest, she decides to keep it as a pet and names it "Tama". At first sight it's just an average cat, but Aria has yet to notice that Tama is a former Knight and that his identity is not a cat, but a young monster of the strongest class "Behemoth".