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.

Sonic the Hedgehog (character) - Wikipedia

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

The Long History of Sonic Prototypes/Unused Content

7 Sonic The Hedgehog Prototypes You Were Never Meant To Play

Brand new Sonic Heroes prototypes uncovered - Tails' Channel

Sonic the Hedgehog/History and appearances (IDW), Sonic Wiki Zone

Sonic the Hedgehog 1 Prototype Livestream

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

Sonic the Hedgehog, Mobius Encyclopaedia

Sonic the Hedgehog (character), Ultimate Pop Culture Wiki

Sonic the Hedgehog 1 Prototype Livestream