Solana this is the thread you want to go to for the Thundering Serpent Hatchling. You must be at least revered in a guild who has completed the gold Challenge mode for dungeons.
Mowse has kindly offered people here a way to get the pet so check out this thread.
Sorry I can't figure out how to post the link to the thread here LOL but the guild is named - Deal With It - on the Shadow Moon server - Horde guild - Mowse is the GM - check out the thread for the best times to whisper her for an invite.
Re: [US Horde] Guild offering Thundering Serpent Hatchling