There are currently two vendor pets (in addition to the four Celestial companions) coming in 5.4. The Harmonious Porcupette, is sold by Mistweaver Ku for 7500 Timeless Coins. This is no surprise since we'll be collecting many Timeless Coins in general while on the isle.
The other vendor pet requires additional items, currency, and PVP. Not pet PVP. Player PVP. World PVP. Killing your friends and the strangers of the same and opposite faction. Yikes.
Speaker Gulan sells the Vengeful Porcupette for 100 Bloody Coins. To earn these coins you'll need to purchase Censer of Eternal Agony from Gulan for 2500 Timeless Coins. After using this item, all players (both horde and alliance) become hostile to you, and killing them will earn you Bloody Coins. Perculia and Olivia Grace confirmed that it's one coin per kill. More details on how this item works here.
For PVP servers, this won't really be any different than it currently is, except that there will just be more people to kill/trying to kill you since your faction will also be hostile towards you. It gives people even more reason to gank/kill you heh.
For PVE servers, this seems like a nightmare. Players who are normally unaccustomed to being randomly killed or griefed will suddenly feel the full effect of a PVP server. Those trying to kill a rare mob will need to watch their backs for anyone using a Censer of Eternal Agony, since it will now be possible for players to kill anyone fighting the rarespawn, and then essentially swipe the rare from them. This may not be a huge concern, though, since many rare spawns can be tagged by multiple people now.
Still, think of how miserable it would be if players were mid-battle with Ordos, and a small group decides it would be funny to go over and disrupt the entire raid. Not for any other reason than "because it's funny". Blizzard doesn't have a strong stance on this, but to me (and many others), it IS griefing. Will this item give people the ability to harass others on a more regular basis?
I'm definitely not for this method of obtaining a pet, and I don't know how on earth I'm going to manage this one. I guess I'll have to find some buddies that will allow me to trade kills with. Unless, of course, they implement a mechanic that prevents kill trading. Ugh, I don't even want to imagine it.
I'm normally behind Blizzard on most of what they design, but this is just one pet implementation that I can't really get behind. I'll wait and see how it goes once it's released, but the general idea makes me cringe.