There are many rarities: unknown, very common, common, uncommon, rare, very rare, and OMG so rare. Yours is uncommon, so he's harder to find then a common, but easier to find then a rare. Therefore he's worth more than a common, but worth less than a rare.
There are generally more common pets than uncommon ones, but getting an uncommon horse isn't uncommon at all. However, getting rares is quite difficult.
I hope that made some sense X3