Werewolves are a monster type in The Elder Scrolls Online
You fight them in quests such as a Werewolf Invasion quest
You can also become a werewolf yourself if you take physical damage from one. You must be taking hits until you see a prompt you have been infected with Sanies Lupinus. While you can be cured of the infection, you can always contract it again at a later date.
After you do, head to Riften.
 Werewolf Spawning Time
Werewolves do not constantly appear on the world map in their locations. They appear in packs of 3. Once they are killed, they will not spawn again that night.
It is rumored the werewolves appear once every 8 days. When they show, they appear for a day and a half. Look at the moons and see if one or both are full. That is a fair indication if the Werewolves are appearing. Because of this it is poor form, in game, to kill Werewolves in their spawn areas.
 Werewolf Quest
Report Locations of Werewolves here! Remember these are Night-time only enemies!