The good folks over at Wowhead put together an excellent collection of all the datamined changes to Hunter Pets in BfAz. It represents a big change from the way pets have been done recently, and even a change in the direction that it had seemed Blizzard was taking them. Essentially, pets seem to be going back to having a defined roll, with each pet family being available with one of Tenacity, Cunning or Ferocity. However, the specs don't really mean what they used to.
Pet Specs
Ferocity
Ferocity Pets give you and your pet 3% leech, and gives you access to the Primal Rage ability, which is a new name for Ancient Hysteria. As they currently exist, there's no real DPS benefit to taking Ferocity pets, however, as Spiked Collar has been added to all specs, which should mean that hunters won't need to exclusively bring Ferocity pets to raid, like in the days of yore.Assuming the DPS to HP ratio sticks around somewhere around 1:5, as it currently is (remember, the relative amount of HP was roughly doubled in our last stat squish), then 3% Leech is a little more than half a percent of your total health each second, then even less than that for SV/BM hunters, as leach doesn't work as you'd expect it to with pet damage.
Cunning
Cunning pets provide an 8% movement speed buff, as well as giving you access to the Master's Call ability, which has been exclusive to PvP all through Legion. I love movement speed anywhere I can get it, though we're still yet to see if this will stack (there's no reason to believe it won't, from the information we have so far), assuming it doesn't, this may be a nice little boost for MM hunters who will be hardcasting Aimed Shots, and need to get where they're going quickly.
Master's Call will be enough to keep this as the primary spec for PvP Hunters, as they tend to always need access to root breaks. In PvE, as long as we have Post Haste, it's hard to imagine many situations where Master's Call will really come in handy (though back when it was available in PvE, I do recall a few raid bosses where I used it).
Tenacity
Tenacity pets give the pet and Hunter 10% increased max HP, and gives us a new ability, Survival of the Fittest, which is essentially the Druid ability Ironbark, but with half the duration and twice the CD.
At the moment, I'm quite frustrated by how squishy Hunters are in the live version of the game (if you haven't recently, take a look at this, where both BM and MM make up the very bottom of the list, by a huge margin); so it could be that I'm just reacting to that, but I'm fairly excited by both of these abilities. Of course, we don't know how damage will be balanced in the new expansion, but right now, I'm often dying to slight over-kills that I could have survived with 10% more health. This won't give us crazy high HP like a warlock, and the SotF DR isn't enough to cheese mechanics, but still, I'd love to have these right now.
The Biggest Complaint
The biggest complaint I've heard so far in the various discord channels that discuss this sort of thing, is that Spirit Beasts are now Tenacity spec'ed. To anyone who's raided as a Hunter at any other point in the game, the notion of raiding as anything other than Ferocity is nearly inconceivable, so it sounds like this change means that you'll no longer be able to raid with a Spirit Beast.
It's hard to guess what direction Blizzard is going with the pet specs this early in the alpha, but I my prediction is Ferocity will not be mandatory for Raiding. If it is indeed the plan to make Spiked Collar and Dash apply across all three specs, just like Combat Experience, then there won't be any DPS gain by going Ferocity. You'll essentially be choosing your spec based on the buffs it gives to you, and/or your party (in the case of brez'ing or Primal Rage).
That should mean that not only will your Spirit Beast still be allowed in raid, or anywhere else you want to use it, but it will likely (from what we can see so far), be the best option, with an extra Tranq Shot-like ability, it's usual heal, and the survivability abilities that all Tenacity pets are going to bring.
The Real Issue
The real problem is, I think, that there isn't really any sort of benefit to going Ferocity. I mean, ok, 3% leech isn't nothing, but it's not really great. If you've got a healer with you, I'd take 10% more HP and a DR any day. There's really no identity for Ferocity with the new specs: Cunning is still the PvP spec, Tenacity is still the tankiest, and Ferocity is now... I don't know, really just "the one with Bloodlust".
I'm not sure if that's a bad thing or not. I'd like to see some sort of meaningful utility added to Ferocity pets, but maybe having higher DPS than the other specs isn't the right answer.
While I can't say I disliked the convenience of being able to change any pet to any spec I wanted in Legion, it was still a bit of a mess. We saw many pet families that didn't have any unique abilities after the Ability Prune, which remained a problem for waaaaaay too long. And really it took away all of the RP aspect of pets. While during raid, I am of course willing to play with an untextured cube if it does the most damage, I don't necessarily think that's what's best for the game. A huge number of people play Hunter for the relationship they have with their pets. I'm not advocating that we go back to the days of Hunter pets acting like in game Tamagotchis, but it doesn't really make sense that they're so well trained they can swap back and forth from their specs the way players can. What I'm saying is, I like the idea of each class family being stuck with one specialization.
However, once you give players a convenience like we've had with being able to swap specs, you can't just take it away. So the only option left, is making the specs not affect your DPS out-put, which seems to be what they're trying to do.
I find it hardest to decide between Arcturis and Loque'nahak
It's fully possible this will change in the next build, or that I've misunderstood the abilities from the datamining, in which case I'll have to eat my words here, but I think we're moving in the a decent direction. I, of course, fully support everyone making their opinions on this known in forums or wherever it is that people attempt to influence the devs, but at least as far as I can tell, they're not going to stop anyone from using Spirit Beasts.
If all of that is past us, what I'm really curious about is how they're going to keep Lone Wolf Mark Hunters from having access to everything. Because we will be able to call pets, and just lose our Lone Wolf buff, in BfAz, there's nothing currently stopping us from keeping our pets stacked to give us access to whatever we need in just a couple seconds. If all the Mages and Shamans die before Bloodlust, pull out a Ferocity pet. If everyone who can battle-rez is dead, pull up a brez pet. Perhaps it's enough of a handicap on having more options to lose a sixth of your damage and have to waste time summoning and dismissing a pet, but I'm kind of counting on someone much more creating than I to come up with some way to use this to cheese something.