They pretty much consistently trade for their rarity and release value, which tends to be another pet of the same rarity released within ~3 months of them.
I imagine the birthday dog would be a welcome wishlist offer to a lot of people because it hits a lot of design themes that are popular, but none of them have any above average demand that increases their value beyond other pets from their release.
Dunno if you needed the dates, but in order they're staff bdays 2009 (May), staff bday 2010 (february) and Kwanzaa 2010^^