A Wolpertinger is small hare-like critter that has the head, body, feet, and tail of a hare with bird wings, antlers, and fangs. One must be drunk in order to see them. They are attracted to the scents of the Brewfest. A drunk hunter can track them using beast tracking.
The free-roaming wolpertingers are called Wild Wolpertingers.
As a non-combat pet
At Brewfest 2008, for 50 you can buy which contains a Wolpertinger companion. (In Brewfest 2007 it was a quest reward.) You do not need to be drunk to see your own or a party member's Wolpertinger, though one still needs to be drunk to see another non-party member's Wolpertinger.
It may also be a reference to a series of books by the author Walter Moers