http://info.sonicretro.org/Stella_the_Cat

"Stella the Cat" is a fan-made ROM of the game "Sonic the Hedgehog" for the Sega Mega Drive/Genesis, featuring a fan-made character called Stella the Cat (aka Stella Hoshine*). She replaces the main character, Sonic.

The author of the hack had planned to alter the levels further with palette recoloring and level redesign but instead decided against working on the hack further.