Nightmare Dib has only made one appearance throughout the course of the series, in "Halloween Spectacular of Spooky Doom". He spent the majority of his screentime in the shadows, only coming into the light once.
He was seen wearing a similar straightjacket that the real Dib was wearing at the time, and while he had the same scythe-like hairstyle, his head was deformed and even larger than Dib's and sported a scar on his forehead.
Nightmare Dib told Dib the prophecy of the "one whose neck blinks" (which he read on a bathroom wall), and that the entire Halloweenie world was really inside Dib's head. He also told Dib about the Halloweenies' plan to escape through his head.