User blog comment:Grimmival/Is Hank a Grimm?/@comment-108.34.42.71-20120507013747/@comment-4563264-20120508005056

Monroe said in the early episodes that occasionally, a Wesen will completely lose control of his... well, his control, and humans will see them for what they really are (hence the werewolf, goblin spider, dragon, troll, ogre, witch, and satyr stories). It takes less of a loss of control for Grimms to see them.