Oracle Text

Transform all Humans. Prevent all combat damage that would be dealt this turn by creatures other than Werewolves and Wolves. (Only double-faced cards can be transformed.)

Card Rulings

9/22/2011 Moonmist causes any double-faced Human to transform, not just Werewolves.
9/22/2011 Whether or not a creature is a Werewolf or a Wolf is checked only as combat damage is dealt. If the creature isn’t a Werewolf or a Wolf at that time, its combat damage will be prevented.
9/22/2011 Moonmist will prevent combat damage dealt by a creature that isn’t a Werewolf or a Wolf even if that creature wasn’t on the battlefield (or was a Werewolf or a Wolf) when Moonmist resolved.