Tsuiho Sareta Ossan Tanya shi, Naze ka Densetsu no Daimeiko ni naru

The elven girl Aria is a stunning and swift adventurer. When a small cat-like beast with an adorable face embraces Aria's fruitful chest during a dungeon exploration, 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 reincarnated chivalrous former Knight and that its identity is not that of a cat, but of a young monster of the strongest class "Behemoth".