Just a heads up, this seems to only be used in north Wales. I'm from the south and I've never heard it in my life before.
Nightie wasn't accepted either (although given as an alternative on a previous page).
I always learned it to write in two words (night gown, night dress)—but they are not accepted. Is it by mistake or I've learnt the XVIII. century English once again?
If you type 'night gown' it accepts the answer but flags up the space as a typo, so there is no 'my answer should be accepted' box with which to report it.
Croeso. Coban/gŵn nos is like a dress a girl wears to bed. Pyjamas are more like trousers and a top.