Tsuiho Sareta Ossan Tanya shi, Naze ka Densetsu no Daimeiko ni naru is a Manga/Manhwa/Manhua in english 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

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”.