Wanwan Monogatari: Kanemochi no Inu ni Shite to wa Itta ga, Fenrir ni Shiro to wa Ittenee! (Koikuchi, Kiki)

The story centers on a man who dies due to overwork, a phenomenon that has unfortunately become a known problem in Japan. Just before dying, the man thought that if he was reincarnated, he would want to live a lazy life of a rich man's dog.

His wish is granted, but it doesn't turn out the way he anticipated. He comes back to life in another world as Routa, a dog who belongs to the daughter of an extremely wealthy family. However, after just one month he grows so much, he is easily larger than a human, and he realizes he is no ordinary dog...but he still decides to pretend to be a dog at all costs!

(Source: MangaDex, edited)