Yaoi and Yuri If Dogs Could Also Walk

Or, as it called in japanese, "Inu Mo Arukeba" by Kazusa Takashima. This is a good story, I liked it. It is 3 chapters (not sure if that's all or not, but seems like it is).

Story:
A high school kid found a dog and took care of him. Further adventures in their life start as the high schooler finds the 'strange' side of the dog.

Spoiler: the 'strange' side of the dog

The dog turns into a human when he's excited

That's the introduction pretty much