@CKarner The ability to buy the hatchling is going away in 6.0, so buy it now.- Jonathan LeCraft (@TheCrafticus) September 17, 2014
Players might not know about this sparky little Battle Pet, especially if they've recently joined a guild. The Thundering Serpent Hatchling isn't rewarded solely to the people that earned the achievement for the guild, it's a pet that is unlocked on the Guild Vendor. Anyone with at least Revered guild reputation can purchase it, but the serpent cannot at this time be caged -- which means you aren't going to find this little guy floating around on the Auction House. If you haven't picked him up yet, you might want to do so now -- because there's been no mention made of any possible future return.