Mori no Kuma-san, Toumin-chuu - Episode 3
This is the R18 version which includes adult sex scenes! Based on a BL manga by Haruchika.
Nowa is a bear who lives in the forest. He found a puppy named Airi a few years ago, and is now raising him. When Nowa goes into hibernation for the winter, Airi is still but a pup. But when Nowa wakes up from his hibernation, he finds Airi has grown into an adult dog.