Hello, the title explains it. My sister's interested in getting a Nightmare Noktoa, is there a way to get it?

Nightmare Pets can be created with a Nightmare Potion

You can either create a new Noktoa or you can change an already existing Pet into a Noktoa with the help of a Noktoa Elixir. They come in many colors such as Common, Dusk or Golden.

Note: The Pet has to be a Noktoa already when you use the Nightmare Potion on it, otherwise it'll become the Nightmare Pet of the species it currently is and there's no cheap way to fix that.

The FAQ Guide that is linked in the upper right of the News page answers many questions about Subeta, including all the ways to color pets and change their species.