Bone (cancelled film adaptations of Jeff Smith comic series; 1998-2000; 2008-2016)
Bone is a fantasy comic book series created by Jeff Smith and published by Cartoon Books, that ran from 1991 to 2004 for 55 issues. It tells the story of the Bone Cousins (Fone, Phoney, and Smiley) who are ran out of Boneville and end up in The Valley, where they meet a young woman named Thorn and her grandmother, Gran'ma Ben. Throughout the series, the cousins try to get home while being caught in trials and tribulations of the valley and eventually a war. The series has become a beloved cult classic, but what many may not know is that an animated film had been attempted a few times, before being shelved.
Nickelodeon Movies/Paramount Pictures version (1998 - 2000)
In August 1998, Jeff Smith signed a deal with Paramount Pictures and Nickelodeon Movies to produce an animated adaptation of Bone.  A first draft script was written a year later, with Smith saying a possible pre-production to happen in 2000 and him being the director.  The story would've followed the Bone Cousins ending up in the valley, meet Thorn and Grand'ma Ben, enter the cow race, and face off and defeat the rat creatures, with a complete yet open-ended ending for possible sequels for the rest of the saga. However, Nickelodeon wanted to change the ending and make the film more kid-friendly, with the Bones to be played by children and focus more on Thorn instead. They also insisted on including pop songs by NSYNC and Britney Spears in the film's soundtrack as well.  The executive meddling caused Smith to walk out of the film, causing it to be cancelled in August 2000. 
Warner Bros. version (2008 - 2016)
Nearly eight years after the previous version's cancellation in March 2008, it was revealed that a new Bone film would be underway, this time by Warner Bros. Pictures with Dan Lin producing.  It was later announced to get a CGI trilogy, with a four minute animation demo reel being completed with Smith detailing the demo as "Fone Bone was falling in the water and going through cliffs and canyons. The dragon moved in from off camera in the shadows with smoke around him, all in 3-D. It was pretty mind-blowing."  Animal Logic (known for their Oscar-winning film Happy Feet) and Lin Pictures were to produce, with Greek creator Patrick Sean Smith directing and P.J. Hogan writing.  But no further details surfaced, and in November 2016, Mark Osborne (Kung Fu Panda) and Adam Kline were to direct and write the still planned CGI film trilogy, effectively replacing Smith and Hogan.  No further news came to be, and was quietly cancelled by October 2019 as a Bone animated series on Netflix was confirmed to be in development. 
As of 2020, no material including concept art, scripts, and footage has surfaced from either version, or if Paramount, Warner Bros., and Jeff Smith have held onto any if they exist. The Warners Bros demo reel animation made in 2011 has yet to be released to the public and it's unknown if animation was done for the film when it was at Paramount and Nickelodeon. As Netflix now owns the adaptation rights to Bone, it's unlikely the film will ever see the light of day at either studio.
- nickslimepodcast's video on the Nick version of the Bone movie as part of their Nicksmissal III series [Episode 5.]
- Ash the Canadian's video on the Nickelodeon version of Bone, including an unproven rumor of test animation by Nick.
- Jay Wallfish's video on the Warner Bros. version.