The chibis are aligned to match the Chinese zodiacs as well as possible. Even the order is correct:

• Rat: Nazrin
• Ox: Keine
• Tiger: Shou
• Hare: Tewi
• Dragon: Meiling
• Snake: Kanako
• Horse: Komachi
• Ram: Patchouli
• Monkey: Iku
• Rooster: Mystia
• Dog: Momiji
• Pig: Yuugi

Obviously some of them are fillers due to lack of better alternatives. Maybe they’ve been selected according to the animals’ characteristics. (→Wikipedia article)

I guess that makes Marisa the cat and Reimu the riceball.

Mokou fits more as a sheep if you know what I mean.