Some new datamining by MMO-C suggests that the reason hunters are getting 50 stable slots in patch 5.3 is because Blizzard is introducing troves of new spirit beasts.
So far 30 new spirit beast models have been datamined by MMO-C, including the spirit turtle, spirit worm, and yes, a spirit Direhorn. Right now we don’t yet know what the spawn rates are going to be — whether these will be rare spawns like the other spirit beasts (in which case pet collecting hunters are in for a world of hurt) or if there will be some new mechanic/quest in 5.3 to get them. Some users on Petopia have suggested that they might be some kind of reputation rewards.
One of the most interesting theories being floated on Petopia is that they are just going ahead and making a spirit beast version of every hunter pet model — that they have some kind of “convert artwork to spirit beast version” macro now and are working their way through the roster.
As always, this is just the PTR and Ghostcrawler has not promised you a spirit direhorn. But the files are all there, so we have assume they’re being used — plus it explains the stable slot expansion.