Read I'm an S-Rank "Behemoth" Monster, but I'm Living as a Knight (Pet) of an Elf Girl Manga:
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 "Beh more.. emoth".