Summer Days with Coo
This award-winning film has been recognized for excellence in animation and storytelling.
Kouichi Uehara is a fourth grade student living in the suburb of Tokyo. One day, he picks up a large stone which turns out to be a fossil of a baby "Kappa" who has been sleeping underground for the past 300 years, and names it "Coo." They become good friends and Coo starts living with Kouichi's family. However, Coo has a hard time adjusting to life among humans and misses his kind.
Plot summary from MyAnimeList. Data and images via Jikan API.