werewolf (n.)

late Old English werewulf "person with the power to turn into a wolf," from wer "man, male person" (from PIE root *wi-ro- "man") + wulf (see wolf (n.); also see here for a short discussion of the mythology). Belief in them was widespread in the Middle Ages. Similar formation in Middle Dutch weerwolf, Old High German werwolf, Swedish varulf. In the ancient Persian calendar, the eighth month (October-November) was Varkazana-, literally "(Month of the) Wolf-Men."