List of Mario & Sonic at the Olympic Games pre-release and unused content
From the Super Mario Wiki
This is a list of pre-release and unused content for the game Mario & Sonic at the Olympic Games.
Birdo was originally going to be a playable character in the game, but appears to have been cut out for unknown reasons. Note that because her model wasn't finished properly, Birdo's nose is inverted in the screenshot. Silver the Hedgehog from Sonic the Hedgehog 2006 was seen in the intro of the game, making fans believe that Silver was meant to be playable. Donkey Kong and Jet the Hawk, from Sonic Riders, have also been found to have been planned for the game. Espio, Rouge, Cream, Big, a Goomba, an Egg Pawn, and an Egg Flapper were also going to appear as playable characters. These characters appear in the sequel, Mario & Sonic at the Olympic Winter Games. Donkey Kong and Silver appear as playable characters, while Jet the Hawk and Rouge appear as bosses and Birdo a VIP Member. Cream was changed to a referee. The Goomba makes various cameos in the crowd in both games. Birdo, Jet, Espio, Rouge, and Cream later appeared as playable characters in Mario & Sonic at the Rio 2016 Olympic Games. In the Wii U version of Mario & Sonic at the Rio 2016 Olympic Games, Rugby Sevens is available, and in it, Egg Pawns can be played as (with Boom Booms playing the same role on the opposite team).
In the demo, there is a section of the game that originally says "Character Select" before changing the phrase into "Select Character". The reason is unknown.