Corrected entry: Through out the Megarace the purple vehicle changes from a tall one wheeled bikey thing to a large four wheeled vehicle.
Correction: We don't get to see all of the vehicles at the start. Several times throughout the race, the racers crash and trade vehicles. The 2 options I can think of is that when racers respawn, they have a small choice of vehicles (as several crashed players come back into the race somehow), or there were power-ups along the way that could add enhancements to vehicles.
Correction: The vehicle is shown changing - that is part of its capabilities.