On the single pet view pages  there is a block called "My Collection". It currently shows a check box for your ownership of the pet and a percentage of users who own the pet.
A useful addition to this would be a list of each copy of this pet that I own, with their levels, rarities, and breeds. A common use for this would be building theoretical pet battle teams and wanting to quickly see what versions of a pet I already have access to.
 Example: http://www.warcraftpets.com/wow-pets/dr ... hatchling/