Mouse is an enemy Badnik encountered in Gene Gadget Zone, the sixth level of Sonic 3D: Flickies' Island.
A trundling mouse-bot, this little machine follows Sonic around when it gets the hedgehog in its sights. While it has no active defensive or offensive weaponry, the Badnik makes every effort to simply drive into Sonic and make him lose all his Rings. Attack with a Spin Dash or jump, and smash it to recover its organic battery power source, a green Flicky, and make Flickies' Island a slightly less dangerous place.
The name of this Badnik is conjectural, as no official name has been given to it. The name comes from the level editor in a development build of Sonic 3D that is included as a bonus feature in Sonic 3D: Director's Cut. Jon Burton has confirmed that the name was used in the original game.
A vaguely similar mouse-bot is the Technosqueek from Flying Battery Zone in Sonic & Knuckles.