When Gale Racer was released, it became apparent that the original promotional screens were mock-ups that bared no relation to the appalling graphics of the actual game. Worse still, the port was missing all the playability of the arcade original, causing it to be one of the most disappointing titles of the initial Saturn line-up. According to one magazine review, the only highlight was the Sonic doll that hanged from the driver’s mirror.
It seems the doll is actually an elaborate Easter egg, which until now, remained unknown. Hari Hari was recently playing the game and noticed that each time 100 races are completed*, the doll changes into another character from the Sonic series. There are 11 characters in total which include all the Mega Drive robot Sonics and even Ray and Mighty. Best of all, because each doll continually spins around (even when the car isn't moving) it is possible to see the characters from a variety of angles.
The characters appear in the following order (Thanks to Hari Hari for the pictures):

Sonic (default)

Tails

Metal Sonic

Mighty

Eggman

Mecha Sonic (silver)

Ray

Amy

Mecha Sonic (blue)

Knuckles


Super Sonic
* There's a small chance that I might have understood that incorrectly...