Hamtaro (ハム太郎, Hamutarō) appears in Story of Seasons: Trio of Towns (Japanese version only).
Hamtaro is a hamster from the franchise with the same name. He is the first obtainable pet who is a character from popular media.
One of his functions seems to be helping harvest sunflowers.
Here is a video showing some gameplay with Hamtaro as your pet: http://www.nicovideo.jp/watch/sm29187485