When patch 5.1 was released, there was a bug with the Darkmoon Hatchling. This caused the pet to be uncommon (green) quality when learned, unlike its fellow DMF vendor pets which are rare (blue).
A fix for this was released, but it only affects the hatchlings that are learned AFTER the fix.
Any Darkmoon Hatchlings that were purchased and learned prior to this hotfix will remain uncommon, according to Mumper. These cannot be changed after the fact.
You can try to put in a ticket to see if a kind GM will refund your tickets so you can repurchase the pet, but it's not guaranteed that a GM will assist. It all depends on which GM you speak with, what their mood is, have they slept well, what time of day it is, which day, if they had their afternoon snack and nap yet...... yea.
It's pretty disheartening news, but I guess I'll try my luck with a GM.
Maybe it's my slight OCD. Maybe I'm just picky and anal about trivial things. But I'd prefer it if all similarly obtained pets stay in the same boat.
Plus I feel slightly cheated that I have to pay the price (spend another 90 tickets if a GM won't refund them for me) for something that was not my fault nor could I have known about it beforehand. :\