Undocumented Changes to Prestige Pets
After the release of Patch 7.3, a change to Prestige Pets has come to light. Thanks to Eggs (EU) and Gráinne of Warcraft Pets, word has started getting out about the ability to purchase the prestige pets on your opposite faction.
The pets cost 500 gold each and require only that you have unlocked the pet on one character, and that you can get an opposite faction character to new Dalaran. You can purchase the pets from Captain Roberts for Alliance and Sarah the Savage for Horde.