The December 18th pets (and December 24th pets this year) are re-releases, which means they're older pets that are being released again on that one day in the big present box. :3 Because they're old pets, they're often much more valuable than current pets or other Advent pets, and the present box can have pets from 2009 or 2008 in there, including Very Rares and OMG so rares, plus unreleased (UR) pets that are only available one day of the year (or two, this year) through that box. That's why it's a very good idea to find out what your Dec. 18th pets are and what their values are before trading them, especially if you're getting a lot of trade offers on them or people asking about them. ;3 There's the 'What is This Pet' thread (
viewtopic.php?f=20&t=25762) where you can ask for an ID on a pet, and some people will identify pets through PM, too. You can also look through the Archives to identify your pets yourself, but that can be very time-consuming if you don't have a general idea of when they were first released.