RC X was an unused logo in the Arcade version of Re-Volt. Its existence may raise a number of questions about its final purpose, mainly due to the fact that the logo presents a car name which was never seen in any version of the game.
One of the questions to be raised is if the logo was really related to an unreleased car that would be exclusive to the Arcade version. For now, there's nothing to prove this statement.
The revolter MightyCucumber created a custom car released at November 2016 with its name based in the arcade logo, RC X, where Mystery was used as base for construction to make fun in the logo mysteriousness.