Right now the rarities are:
OMG so rare! (less than 1 pet per 1000 users)
Very rare (less than 1 pet per 80 users)
Rare (less than 1 pet per 25 users)
Uncommon (less than 1 pet per 5.5 users)
Common (less than 1 pet per 3 users)
Very common (more than 1 pet per 3 users)
we could potentially estimate how many of one pet exists based on its rarity. It wouldn't be too accurate, especially since these numbers are from 2009, but I think it'd be easier to appreciate the rarity of some pets if we could guess about how many of them existed on this site.