There are two different kinds of Worgen on Azeroth. The first, and most common kind have been summoned. They are found both on Kalimdor and the Eastern Kingdoms. In the past, most worgen were summoned by wizards, for it seems that there are links between our world and theirs, and powerful magic can open those links and allow them through. Most folklore, myths and village tales stem from these worgen of old, summoned by wizards whose names have been lost to time. What happened exactly after that is unknown.