Rides across the stage and attacks anything in its path.
Excitebike (エキサイトバイク) is the brand of eponymous motorcycles from the NES video game, Excitebike (part of the Excite franchise) that appears with the racers from the game as an Assist Trophy in Super Smash Flash 2.
Excitebikes are, for the most part, typical motorcycles that are used in Excitebike racing competitions. As they are used for sport, they have very high specifications and can perform high-profile maneuvers such as launching high in the sky off ramps. Each Excitebike is equipped with a turbo-boosting system which allows it to speed up to very high speeds, but puts a strain on the overheating engine if used for extended periods of time. On each racetrack are icons that instantly cool down your engine if you ride over them, though.
In Super Smash Flash 2
Like with its appearance as an Assist Trophy in Super Smash Bros. Brawl, the Assist Trophy itself consists of a horde of Excitebike racers appearing in the same sprite form they did in the original Excitebike for NES. They race around the stage and damage players who touch them until eventually riding off the stage or, if enough time passes, disappearing. All the different original sprites from the NES game appear to race across the field, and which ones or how many appear with each summon is always random.
As seen on the art above, there is an uncommon Excitebike racer with a black and gray color palette that may appear alongside the other racers. This biker is an homage and reference to the Mach Rider, a motorcyclist from the NES game of the same name.