Winsor McKay didn't just create one of the most influential comics of all time, Little Nemo in Slumberland — he also was an animation pioneer, creating some of the first real animated films. So it's kind of sad that the animated movie version of Little Nemo, from 1992, was such a flop — at one point, I saw it listed as the most unsuccessful comic-book adaptation ever released in theaters. But at the moment, it's being beaten by Dylan Dog: Dead of Night and Return of the Swamp Thing.
In any case, Little Nemo: Adventures in Slumberland is actually not a terrible film. According to one IMDB review, Hiyao Miyazaki, Moebius and Brian Froud all worked on it, which is a major plus. This Japanese-U.S. coproduction is a fun animated fantasy adventure, which keeps at least some of the iconography of McKay's comic. It does take lots of liberties, and I'm not sure the dark nightmare villain comes from the comics. It would be hard to match the level of surrealism and raw weirdness that McKay packed into every page of his comic, and the animated film instead settles for a kind of gentle Seussian strangeness, which meshes pretty well with the whole "dreamland" aesthetic.
In any case, the whole thing is on Youtube. Here's the first part: