Tornado

The Tornado was originally Sonic's bi-plane that he traveled to South Island in to search for the Chaos Emeralds, later on in the series it becomes Tails' airplane. By the time Sonic Adventure came around it was capable of multiple transformations. It originally appeared in Sonic 2, but reappeared in Sonic 3,Sonic and Knuckles, Sonic Advance and Sonic Adventure. Over the course of this time, it has gone over major revisions, starting out as a biplane, turning into a multicapable futuristic force, driven by a chaos emerald. The Tornado did not appear in Sonic Adventure 2, instead being replaced by the Cyclone.