I AM BEHEMOTH OF THE S RANK MONSTER BUT I AM MISTAKEN AS A CAT AND I LIVE AS A PET OF ELF GIRL is a Manga/Manhwa/Manhua in (English/Raw) language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

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".