Some information about zebras:
There are three zebra variations: Plains, mountain, and Grevy's. Zebras have been (and will be) released with every Equimagine pet since August 2009, excluding the BHA horses. These are the rarest CS pets to date, as only one will be adopted for approximately every 2,700 people adopting to the limit per set of horses, or around one in every 40,000 adoptions, though this statistic varies slightly from set to set. Zebras are so rare because Tess loves them and wants to keep them all to herself XD
There will only ever be three zebra variations-- each variation will be released multiple times; there is not a different zebra for every horse release.
The three different zebras look like this: (I suggest you save the images for each stage)

Also, if you keep track of these three you can document the days they grow :3
On the "unreleased" pets:
The four "unreleased" pets were pets only available on December 18, 2008, and are the rarest pets aside from zebras. The unreleased horse and butterfly wolf were designs that had existed for a while but had never been put up before, while the cat design used to be someone's custom before they were banned. The banana was completely random and began yellow with a happy face but quickly "rotted". Some of the bananas were deleted after they were adopted because the members who got them didn't want fruit in their pet collection XD
It is extremely unlikely that these four pets will ever be released again, but it's up to Tess...