What animal is Pikachu based on?

Pikachu, the yellow Pokemon that Ash Ketchum typically carries with him, is listed on the official Pokemon website as a "Mouse Pokemon," though anatomically, Pikachu is probably more remniscent of a hamster or a pika.

The similarity between the names pika and Pikachu has led to speculation that perhaps Pikachu is based on the pika. The pika also emits a high-pitched warning whistle that is vaguely similar to Pikachu's characteristic battle cry, though the pika, unlike Pikachu, has no tail. Pikachu evolved from Pichu, who appears even more mouse-like, with large ears and a small, rounded body. Pichu is officially categorized as "Tiny Mouse."