Just something to keep in mind: the Armory profile reflects the quality of the item, not the pet itself. There are many cases where the qualities don't match. For example, the Magical Crawdad will appear in your Armory as a Common item because that's the quality of the item it is learned from. The Magical Crawdad battle pet itself as it appears in your Pet Journal, however, is always rated Rare. In the other direction, the new Imperial Moths that tailors can get come from Rare quality items, but as far as I can tell, the battle pets they create are always Uncommon. Again, the Armory reflects the Rare rating of the item, while the Pet journal tells the real story and rates it as Uncommon. It's confusing and I wish Blizzard would make it so the items and the pets themselves had the same quality, but that's currently how it works.
In the case of the Darkmoon Hatchling, the item you purchase is Rare. However, until they patched it, it was giving out Uncommon pets. To find out for sure which one you have, don't check the Armory. Instead, look in your Pet Journal.