What kind of animal is Chiko?

Question

The pet of Kyuuta (Ren) is a fluffy ball of hair-like animal. Though Kyuuta thinks and says it is a mouse, I didn't find anything to confirm that Chiko is a mouse.

So what animal (if it is an animal) is Chiko?

Answer 1

Officially, Chiko is a mysterious, small animal.

The official site (Japanese) lists Chiko as,

チコ
九太と【渋谷】の路地裏で出会って以来、いつも一緒に行動している不思議な小動物。

Chiko
A mysterious, small animal that always with Kyuuta after their meeting on a back alley in "Shibuya".

^{(Emphasis mine)}

In contrast, the official site lists some characters with their animal characteristic:

• Kumatetsu: Bear (kuma)
• Tatara: Old monkey
• Hyakushuubou: Slim pig

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Out-of-universe, it looks like a Kesaran Pasaran, a Japanese mythical creature (youkai) that looks like a white fluffy furball. While it has many renditions, one of them looks like this:

^{(Image posted by Ein on Serenes Forums)}

^{PS: I only found Answers.com as the best definition in English, but it doesn't conflict with Japanese Wikipedia.}