You must choose an existing creature type, such as Human or Warrior. You could even choose Coward, if you dared. Card types such as artifact and supertypes such as legendary can't be chosen.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to creatures of the chosen type may become lethal if Brass Herald leaves the battlefield during that turn.

The choice of creature type is made as Brass Herald enters the battlefield. Players can't take any actions between the time the choice is made and the time the appropriate creatures begin to get +1/+1.