Withers is bind on pickup so any character wanting this pet will have to make the journey in person. On the way, be sure to swing by The Eye of The Vortex and watch for flying cows! Trust me, it's worth seeing.
Withers is rewarded from a long quest chain in Lor'danel, a new Alliance city in Darkshore. He therefore appears to be an Alliance-only pet. He's awarded to players by a new NPC named Cerellean Whiteclaw for completing the final quest in the series, Remembrance of Auberdine.
We remain confident that the developers will insert a Horde-only equivalent of some sort, possibly even the same pet. We'll keep you posted as we continue to receive reports from beta testers.