You must choose an existing creature type, such as Sliver or Warrior. Card types such as artifact, and supertypes such as legendary or snow, can't be chosen.
Cost reduction effects are applied after other cost modifiers, so Morophon can reduce additional costs or alternative costs of spells of the chosen type.
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to other creatures you control of the chosen type may become lethal if Morophon leaves the battlefield during that turn.