What kind of animal is Chiko? 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?
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 &quot;Shibuya&quot;.
(Emphasis mine)

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

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


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.