Historical Rules · 1 October 2009
Rule 204.3d
Official source text from the 1 October 2009 Rules release
If an instruction requires choosing a subtype, you must choose one, and only one, existing subtype, and the subtype you choose must be for the appropriate card type. For example, you can't choose a land type if an instruction requires choosing a creature type. Example: When choosing a creature type, "Merfolk" or "Wizard" is acceptable, but "Merfolk Wizard" is not. Words like "artifact," "opponent," "Swamp," or "truck" can't be chosen because they aren't creature types.