The failure to get the film out in any reasonable amount of time helped bring down Ron Miller's regime at Disney, and its financial failure ensured that he would never work in Hollywood again.
Co-directors Ted Berman and Richard Rich were also fired from Disney in the wake of the film's release; Berman never worked in animation again, while Rich ended up having to start his own studio in order to continue his career.
Inverted with Jeffrey Katzenberg, who ended up becoming more influential within Disney as a result of this films failure; he was able to utilize his "I told you this would flop" position on the movie over the older executives who had believed in it, painting their tastes as out of touch with what current moviegoers wanted to see.
It was very nearly the death of Disney itself! Thankfully, it's spectacular and humiliating failure convinced them to finally get their act together and make better movies.
Dueling Movies: With The Care Bears Movie, its polar opposite. It marked the first time that a Disney animated film had gone head-to-head with a non-Disney animated film at the North American box office and lost.
It jumps straight into a very-loose adaption of the second story of The Chronicles of Prydain. Which in of itself is a very, very dangerous move.
Contrary to popular belief, the infamous cuts made to the more violent and nightmarish scenes of death, decay, and destruction weren't made because Disney believed cartoons were for kids (far from it. Had the executives not intervened, this would have been Disney's first foray into more teen- and adult-based animation). The reason was because then studio chairman Jeffery Katzenberg didn't know about Disney's plan to make an animated movie that had more adult content in it and forced the producer Joe Hale to cut the film by 12 minutes without being given a thorough explanation first. If the original had stayed, viewers would have seen this fully uncut upon its initial premiere.
Katzenberg originally only wanted ten minutes cut, but the filmmakers only cut six minutes, feeling any more would hurt the story. It turned out that he meant literallyten minutes and, when he discovered that they hadn't met his specific demands, proceeded to personally edit out even more than he originally requested.
Franchise Killer: Disney secured the rights to the entire Chronicles of Prydain, but the film flopped so hard, it killed off any future plans of continuing the series.
Germans Love David Hasselhoff: The Horned King has plenty of attention in Japan. This may have had to do with the extinct attraction in Tokyo Disneyland. Shall we begin?
Mickey Mouse on the Game Boy in 1989 has him as the final boss.
Mickey Mouse II on the same system two years later again has him as the final boss.
Both of these games replaced him with Witch Hazel when they were converted to the Bugs Bunny Crazy Castle games.
Mickey Mouse III: Dream Balloon in 1992 again puts him as the final boss, only to be replaced by Night Mayor when it was converted to Kid Klown in Night Mayor World.
Finally appeared without alterations in Land of Illusion on the Game Gear and Master System in 1993, although this time colored from older concept art and given the name of "The Phantom." And this was the only time that he appeared in North America and Europe in a Japanese produced game.
How did the Japanese dub make him sound scarier? Give him Shozo Iizuka's voice!
Old Shame: Disney went to great lengths to disassociate themselves from the film. For example:
They opened a Black Cauldron-themed restaurant at Disney World ("Gurgi's Munchies and Crunchies") in anticipation of the film's release. Once it bombed, they closed the place down then later remodeled it into a Beauty and the Beast themed establishment (which is now replaced with a Robin Hood establishment).
It was because of The Little Mermaid that this wasn't given a video release sooner — according to the IMDb, Disney was all set to release it when the latter film became a smash hit and they were pressured to get it on video ASAP. Because this movie was both atypical and unpopular, it wasn't released to video in North America until 1998.
Maybe not so much now. The movie received a digitally restored DVD rerelease during its 25th anniversary, which even got promoted on some higher-profile DVDs.
Overlaps with No Export for You: "The Black Cauldron" has never been released or dubbed in Hungary, Turkey, Israel, or Thailand (though that may also be for content reasons, as those countries do treat animated pieces like children's fare).
The Other Marty: Hayley Mills was originally cast in the role of Princess Eilonwy and hosted a behind-the-scenes special on Disney's Wonderful World in which she introduces herself as the voice of said character. Then, for reasons unknown, Mills was replaced.