Tails in Sonic 1 is a hack of Sonic the Hedgehog for the Sega Mega Drive/Genesis originally created by drx which featured Tails as a playable character instead of Sonic. Tails can fly, spindash and swim in the same way that he can in Sonic 3.
On June 8, 2007, Puto released an update to Tails in Sonic 1, which featured several improvements over the original hack: bugfixes, a corrected palette and Sonic 2-styled tired and swimming sprites. More bugfixes have been released since, and on 12/31/07, he released version 2.0, completely rewritten from scratch. Version 2.1 was released on 10/09/08 to no fanfare; its purpose was to fix some differences in Tails' flight behaviour, making it 100% identical to the flight behaviour used in Sonic 3. Version 2.1.1 was released on the next day to fix one more bug.
The patch is in xdelta format due to shifted data resulting in a IPS patch almost as large as the original ROM.
Tails in Sonic 1 v2.1.1 by drx & Puto
Tails in Sonic 1. It says everything. It flies, it spindashes, there's no
sign of Sonic in the game. This is an update by Puto, which fixes alot of
bugs.
The included patch was created using the hacked ROM file available at drx's site.
The unmodified ROM used is listed as Sonic_The_Hedgehog_(W)_(REV00)_[!].bin
Sonic The Hedgehog (USA_ Europe).md - NOINTRO
CRC32: F9394E97
MD5: 1BC674BE034E43C96B86487AC69D9293
SHA-1: 6DDB7DE1E17E7F6CDB88927BD906352030DAA194
SHA-256: 46160BAA06362C711C9F1A5017CB7371026444936C8AF5E93A78996CF32FF2A6