Seton Doubutsuki: Risu no Banner

Soon after birth a young squirrel comes to a farm, and gets caught by a human. This human then tries to find him to their pet cat. However, the cat feels sorry for the squirrel and adopts him as their child and names the squirrel Banner. Banner and the cat live happily in the farm, but their happy lives don't last long. A fire destroys the farm, and Banner gets separated from his mother the cat.

(Source: Anime News Network)