Dream Eater Danjon

This story is about a very shy girl called Nishimaru Katori who has a big crush on a boy in her class called Nanamiyaharu Tooru. When her old sheep dog, Jon, dies it's revealed that he's actually a Baku or nightmare who had himself reborn as a dog because he was bored, and if he manages to make Tooru and Katori fall in love he'll be reborn as their child. (Source: B-U)