Author Avatar: Beagle has stated frequently in interviews that the butterfly who constantly quotes song lyrics is supposed to be him. In a meta-sense, he explains that he, the author, needed a reason for the Unicorn to start her quest, therefore he inserted himself in the form of the butterfly to tell the story of the Red Bull that the unicorn could not have otherwise learned. In the film adaptation, it even has a scruffy beard resembling Peter's own at the time he wrote the book.
Creator Backlash: Beagle has been frank that he found the book to be the most difficult to write of any in his career. It's not that he dislikes the book, but rather that he's intimidated by its success and knows that he'll never get it quite that right again.
Promoted Fanboy: Christopher Lee was a huge fan of the book, to the point of bringing his own copy of the book to his recording and pointing out lines that he felt had to stay in the story. He also provides the voice in the German dub of the movie.
The cat's pirate voice was ad-libbed by his voice actor who was having fun with all sorts of accents to give the cat, and everyone loved the stereotypical Long John Silver voice that they just had to go with it. They gave the cat an eyepatch and peg leg to match.
"Have a taco" came about when Beagle was writing very late at night, when you start writing all sorts of strange things. However, he loved the line so much he had to leave it in, Anachronism Stew be damned.
Wag the Director: A positive example. When Christopher Lee came in to work, he brought his own copy of the novel wherein he took note of lines that he believed should not be omitted.
A live-action remake has been in Development Hell since 2005. Christopher Lee for a long time had expressed interest in reprising his role as King Haggard. Shortly before his death in 2015, he had given his blessing for them to cast someone else. Angela Lansbury had also expressed interest in reprising her role as Mommy Fortuna.
The UK group Renaissance was considered to create the soundtrack before America came on board.