One thing that is worth noting is that some players can summon both the Jade Crane Chick and Thundering Serpent Hatchling on the beta.
The journal's information on the Jade Crane Chick says that it should be available from a vendor in Stormwind and Orgrimmar, but both NPCs don't seem to be selling anything at the moment. This might change in the future once pet battles is officially open for testing.
And as many have already seen first-hand, the Thundering Serpent Hatchling is available for purchase from the guild vendor for 300 gold. What guild achievement unlocks this pet? I still have no idea despite combing through the achievement tab and searching for any possible leads online. Guess we'll just have to wait and see.
Note: Not pictured properly in the screenshot is the pet's flickering lightning aura. It's very faint in the image above, but in person it's much more prominent. This companion also currently has the same idle animations as the Soul of the Aspects. I'm hoping that it will get its own unique animation or interaction with other pets/the environment, yet I wouldn't be surprised if it didn't.