One of Won Won -The One of Wonwon-

Main heroine's ex-boyfriend invites her to visit a dog God shrine. But when they arrive there, he starts to behave mean and claims that person who argues in front of the shrine will turn into a dog. And of course they quarrel over a trifle, and boy turns into a dog. Heroine visits people linked with the shrine to find a cure for the boy.

[From vndbreview]