New Sonic the Hedgehog (1991) prototype uncovered by preservation groups - Tails' Channel

A prototype of Sonic the Hedgehog (1991) was showcased by a video game preservation group in a New Year's Eve livestream.

SEGA Sonic the Hedgehog Prototype Game Build from 1991 Surfaces, Here's a First Look - TechEBlog

7 Sonic The Hedgehog Prototypes You Were Never Meant To Play

Sonic Generations PSP build assets uncovered by preservation group - Tails' Channel

Sonic Forces (Video Game) - TV Tropes

Sonic 3 Prototype With Lost Content Discovered - IGN

Proto:Sonic the Hedgehog (Genesis) - The Cutting Room Floor

Sonic the Hedgehog (Prototype) LOST BITS Altered & Unused Content [TetraBitGaming]

Newbie's Perspective Archie Sonic Reviews Issues 1-29 Deluxe Scripts – CrystalMaiden77

Sonic 1 Archives - Tails' Channel

Sonic Origins, Sonic Wiki Zone

Sonic the Hedgehog (lost Tokyo Toy Show prototype build of Sega Genesis/Mega Drive platformer; 1990) - The Lost Media Wiki