The Thundering Serpent Hatchling is a guild vendor pet that can only be unlocked by completing the MoP Gold Challenge Mode guild achievement. Once the expansion is released, the MoP Challenge Modes and achievement will become a feat of strength.
One thing that Blizzard has failed to mention, though, is that the pet will no longer be available on the guild vendor as well. I've checked on the beta and it's not listed under the guild rewards tab (but the new Draenor gold Challenge Mode pet, Deathwatch Hatchling, is shown). The guild vendor wasn't showing any available items at all, but that could be due to my character not having the appropriate reputation with the guild.
So if the Thundering Serpent Hatchling can't be acquired through a guild anymore, will it be removed entirely? The answer is no. This pet will absolutely still be available in-game, however at a much higher price.
The beta Pet Journal was updated to include where this pet can be purchased from in WoD -- The Black Market Auction House. I haven't been keeping tabs on the BMAH, so I can't say for sure how much the Thundering Serpent Hatchling will cost, but it will probably be more than the current 300 gold.
It's entirely possible that the pet will still be available from a guild vendor, and it was simply an oversight to not include it in the guild rewards list on the beta. But it's also entirely possible that it's being retired to the BMAH.
In light of this, the question that collectors need to ask themselves is whether or not you want to risk it and wait for WoD to collect the Thundering Serpent Hatchling, even though you can collect the pet at a reasonable, pre-determined price right at this very moment. There are a few guilds out there (WarcraftPets listing some) willing to invite collectors just to collect this pet.
For me, if I were still seeking this pet, I wouldn't take the chance and I'd purchase it prior to the expansion.