Sonic the Hedgehog is a video game series and media franchise created and published by the Japanese company Sega. Since the release of the original Sonic the Hedgehog for the Sega Genesis in 1991, several Sonic games have been canceled or reworked into other projects. Builds of unreleased Sonic games have leaked online over the years.
Early in Sonic's history, two spin-off games—the edutainment game Sonic's Edusoft and the falling block puzzle game SegaSonic Bros.—were completed, but Sega declined to publish them. Sega and Nihon Falcom planned to remake Falcom's PC-8801 game Popful Mail (1991) for the Sega CD as a Sonic game, but canceled it in favor of a more faithful remake after a negative fan response. Unreleased Genesis Sonic games include three spin-offs pitched by Sega Technical Institute (STI) and Sonic Crackers, which was reworked into the 32X game Knuckles' Chaotix (1995).
After the release of Sonic & Knuckles in 1994, STI began working on Sonic X-treme, planned as the first Sonic 3D platformer and the first Sonic game for the Sega Saturn. Development was hindered by company politics, problems adapting Sonic to 3D, and crunch. After two lead developers became ill, Sega canceled X-treme and released a Saturn port of the Genesis game Sonic 3D Blast (1996) in its place. The cancellation is considered an important factor in the Saturn's commercial failure, as it left the console with no original Sonic platform game.
After Sega exited the video game console market to become a third-party developer, plans to develop a sequel to Sonic Adventure 2 (2001) were canceled twice, first in favor of Sonic Heroes (2003) and later in favor of Sonic Unleashed (2008). Other unreleased Sonic games include the skateboarding game Sonic Extreme, which may have served as the basis for Sonic Riders (2006), and proposed follow-ups to Sonic Chronicles: The Dark Brotherhood (2008), Sonic the Hedgehog 4: Episode II (2012), and Sonic Mania (2017).