18+ Soko ni Osuwari!

Shuuya Inukai is a model student at a high school who one day attracts the attentions of an ojou-sama, Kantori Isuzu, when she realizes that he is bearing, on his forehead, an X-shaped scar...that was also present on her pet dog's forehead...which leads her believe that Shuuya is actually the reincarnation of the said animal. And now Shuuya's days as Isuzu's pet begin.

(Source: MU)