If you're a battleground hunter like me, it's annoying that your pet does not level with you on battlegrounds. Well, I discovered that in Alterac, killing harpies in the cave below Dun will give both you and your pet XP while the game is going on, plus you still get the normal XP earned from your team-mates capturing AV objectives.
Alterac gives a lot of XP already, you can really get fast XP for both you and your pet by just spending a game or two shooting harpies.