Sonic the Hedgehog

Sonic the Hedgehog was a video game. In 1989, Alistair Gordon Lethbridge-Stewart was aware that the main character was a blue hedgehog. (PROSE: Business Unusual) Mel Joseph was playing the game on his computer, completing the Scrap Brain Zone. (PROSE: Conundrum) Mickey Smith had a copy of the game on either his Playstation or Xbox, along with Grand Theft Auto and Gran Turismo. (PROSE: Winner Takes All)

Ace joked with Dr John Watson that whatever the Rakshassi were worshipping, it wasn't Sonic the Hedgehog. (PROSE: All-Consuming Fire) Martha Jones once described the Tenth Doctor's hair as "very Sonic the Hedgehog", although the Doctor said he was going for more of a Sid Vicious vibe. (PROSE: Forever Autumn)

Behind the scenes
Sonic the Hedgehog is the flagship franchise of Sega.

Sonic appeared alongside the Doctor as a playable character in LEGO Dimensions. Sonic will occasionally interact with the Doctor if they are paired together during gameplay, where it's revealed the hedgehog incorrectly believes the Doctor named the sonic screwdriver after him.

While the character is implied to have been around from as early as 1989 in the DWU, in the real world his first appearance was in 1991.

A number of Sonic or Sonic-related video games have been developed by studios that have also developed Doctor Who games:
 * Traveller's Tales, the company that later merged into TT Games (themselves the developers of LEGO Dimensions), developed and.
 * Sumo Digital developed the games, , , and.

The Sonic franchise has also had many comic book adaptations:
 * A series titled ran in the UK from 1993-2001, for whom Richard Elson was a prominent illustrator, and Lew Stringer a prominent writer. Mike McMahon also did art for the series in its later years.
 * Among the comics most notable exclusive villains were the "Brotherhood of Metallix", an army consisting entirely of s (Sonic's robotic doppelgänger from the video games). Many fans noted similarities between the Brotherhood and Doctor Who's own Dalek Empire, with them even being led by a larger "Emperor Metallix". Nigel Kitching, one of the comics most prolific writers, acknowledged this, stating that he did not intend to "rip off" the Daleks but, after noting the similarity, chose to run with the idea. He further stated that, since Doctor Who was off the air at the time, he believed the similarity was unlikely to jar the series' readership.
 * More recently, began in 2018 under IDW Publishing.
 * A reference to Doctor Who appears in the comics' nineteenth issue, in which E-123 Omega, a robot character, exclaims "EXTERMINATE! EXTERMINATE!"