I'm ngl I see a LOT of different views/opinions on Nightmare's gender identity (And Dream's), so I decided to give my two cents for some reason!! I just like yapping about headcanons ig, lol... PLEASE note that this isn't a criticism of anyone with different views on this, I respect and in fact ADORE the different takes on Nightmare/Dream that I see!
anyways back to the topic. Idk how to describe this because I don't know a lot of the different names for gender identities, but I think Nightmare just... wouldn't care about gender?? Like, they go by he/she/they/any and ALL pronouns. Nightmare is more preoccupied with what outfit is the most comfortable yet elegant and intimidating, not whether it's more feminine or masculine. They're an immortal skeleton guardian being, so how their gender is perceived isn't really on the list off important things to them.
I feel PRETTY much the same way about Dream, honestly. I can see Dream enjoying dressing up in different feminine and masculine outfits. Though I'm not sure whether they just don't really care how their gender is perceived like Nightmare, or if they're more on the genderfluid spectrum. I'm still sitting on it.












