Sold by Fred and George in Weasleys’ Wizard Wheezes, Pygmy Puffs are miniature puffskeins, “round balls of fluff in shades of pink and purple,” that sell so quickly they “can’t breed them fast enough.” (HBP6)
Ginny convinced her mother to buy one for her, and she named it Arnold. Arnold is frequently seen riding around on Ginny's shoulder in the Gryffindor common room, often with a hopeful Crookshanks trailing behind (HBP14).
Possibly inspired by tribbles from Star Trek. -BB