Sonic the Hedgehog was a series consisting primarily of video games released by the Japanese entertainment company SEGA, and a vast variety of spinoff media such as comic books, television series, theatrical films, and other merchandise.
The series follows the adventures of the titular character, an anthropomorphic blue hedgehog who is capable of running at supersonic speeds, who battles alongside his friends and allies against the evil plots of Doctor Ivo "Eggman" Robotnik, who is intent on conquering the world with his robotic minions.
Outside of a single explicit crossover in LEGO Dimensions, the Sonic franchise and the DWU each yield of number of references and shared backgrounds with each other.
Crossovers
Sonic's universe has had one official crossover with the Doctor Who universe via the mass crossover toys-to-life video game LEGO Dimensions. The Sonic franchise was added to the game via its second wave of figurines and adventure packs on 18 November 2016, joining Doctor Who and the other existing franchise elements that were built into the game upon its original release.
This inclusion allowed players to control Sonic and have him, among others, play through the Dalek Extermination of Earth story, and explore the Who-themed adventure world, as well as operate K-9 and interact with the Doctor's TARDIS.
In turn, this allowed the player to play as the Doctor or the Cyberman during the Sonic Dimensions story, and explore Sonic's own adventure world, as well as using the two related vehicles, the Blue Speedster and the Tornado in any world.
Sonic himself has a unique voice clip that plays when he is paired with the Doctor on the game board: "Hey, you're the Doctor, right? I heard you named a screwdriver after me. How cool is that?"
In promotional material related to the game, the two franchises first appeared together in the trailer New Adventures Await!, although they did not sure any scenes together: following the Doctor and the TARDIS making an appearance earlier in the trailer, Sonic appears at the end, where he is seen swiping a ring from Golem.
In Sonic the Hedgehog Joins LEGO Dimensions!, a trailer made up entirely of gameplay clips, a scene occurs in which Sonic appears inside the early 1960s rendition of the TARDIS, where he runs up beside the Twelfth Doctor at his blackboard, with the Doctor turning to the hedgehog and whipping out his Sonic screwdriver in surprise.
References within the DWU
to be added
References to the DWU in Sonic the Hedgehog media
Behind the scenes connections
A handful of Sonic or Sonic-related video games have been developed by the UK-based Traveller's Tales and their eventual Giant Interactive merger TT Games.