Shamelessly copying my post from the official forums for those of you who aren't already aware of the lovely change Blizzard made to our pet specializations:
In case some hunter players are still not aware, as of 7.0.3 your pets all share the same specialization (Tenacity, Ferocity, or Cunning) at any given time; you can no longer assign specific specializations to individual pets. If you want a tank pet for solo content, you have to get out of combat and respec your pet. Going into a dungeon? Respec your pet again.
For me, this really destroys the illusion (and thus, immersion in the game world) that my pets are anything but interchangeable DoTs with legs that happen to have names and unique skins. I've gone from collecting various unique beasts and camping elusive rare spawns for weeks on end to running around with just two pets: a core hound for Ancient Hysteria and a spirit beast for Spirit Mend.
I used to keep a tenacity turtle for emergencies, a cunning spider for spontaneous world PvP ... but that's over. These pets lost their identities and their role in the game the moment their spec(s) became attached to me, the hunter, rather than to the pets themselves. There's no point to having a "tank pet" when it's no more able to do its job -- without a ten-second out-of-combat channel -- than whatever pet I'm already running with. World PvP? Excuse me for a moment while I respec this spider that I summoned. What? You're attacking me already? No, stop, I need to be out of combat to respec my pet!
I'm still optimistic that this is a bug, and that it will be fixed in the coming weeks. I'm enjoying the crap out of 7.0.3 so far on my druid, and this pet spec problem is the only thing preventing full enjoyment of my hunter. But if it was an intended change, a) I implore you to reconsider, and, if not, then b) at least just let the hunter community and myself know so that I may either move on or suck it up and try to find a way to enjoy playing my hunter despite this great step backward in pet diversity.