The original Japanese version is an interesting case: Princess Mononoke is the first Ghibli film that uses celebrities to provide the voice of the main characters of this film; since then, all of the Ghibli films used celebrities to voice the main characters. The only known professional with a named role in the movie is Sumi Shimamoto as Toki, Lady Eboshi's servant.
Cross-Dressing Voices: Moro is voiced by a man, in this case Akihiro Miwa, in the Japanese version. This comes from Japanese mythology which says that wolves are always male-voiced (and cats always female-voiced), regardless of sex.
Executive Meddling: Fortunately averted, as opposed to earlier efforts to adapt Miyazaki's films into English. Harvey and Bob Weinstein of Miramax wanted to remove 20 minutes from the English release, which, given Miramax being a division of Disney at the time, was explicitly forbidden by the Disney/Ghibli distribution agreement. When Studio Ghibli producer Toshio Suzuki found this out after a very successful screening for the New York Film Critics Circle, he allegedly sent the Miramax executives a katana with a note attached: "No cuts."
Keep Circulating the Tapes: Disney's sale of Miramax was in the way of any possible re-release in the US for several years, although Alliance Films in Canada was still able to print its release. Miramax's rights lapsed at some point and Disney re-issued the film (officially re-printing the original DVD release from Buena Vista Home Entertainment re-badged in Disney's Ghibli package design and with almost every mention of Miramax removed) in 2012, with a Blu-ray release in 2014.
The earliest concepts Miyazaki had that involved a Mononoke spirit was actually his re-telling of Beauty and the Beast.
More early concept art details a version of the film starring a prototypical Satsuki and Totoro in ancient Japan; it's likely that this is what provided the basis for Totoro's own film in 1988. This artwork did eventually see a public release in the art book Princess Mononoke: The First Story.
Leonardo DiCaprio was considered to voice Ashitaka in the English dub. Possible Casting Gag since Claire Danes was the voice of San, which would have made this the second time they had been Star-Crossed Lovers.
Miyazaki intended for this film to be his last, but came out of retirement after conceiving the story for Spirited Away.
The first title for the movie would have been "The Legend of Ashitaka".