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This actually may be a bug. Since the copy effect doesn't copy the effect making the card a creature, I don't think it can have a creature subtype. I think this is supposed to be a Token Land--Forest Island.
I am not a judge though. Can someone who is confirm or deny?
Yeah, I think we've forgotten to mark Nightmare as a creature type. I'll fix it today. #wotc_staff
EDIT: Nope, Nightmare is indeed a creature type, but there's apparently something wrong in our application of copy-changes for subtypes. More fun to fix, I'd say!
I'm not convinced this is wrong behavior. Nightmare shepherd doesn't say it returns as a creature
As Filobel replies, the bug is that the land is a "Nightmare" - it should not be. When processing copy changes, we weren't verifying that the changed-subtype matches the new object's cardtypes. #wotc_staff