Revan, the star of Knights of the Old Republic, is a fairly popular character in Star Wars' expanded universe. So it's probably for the best that plans to include him in the woeful Clone Wars cartoon fell through.
As you can see here, Darth Revan (to explain that would be to spoil things) was intended to make an appearance in an episode of the computer-animated cartoon series, called "Ghosts of Mortis".
With his appearance slightly tweaked to match the "action figure" style of the Clone Wars cartoon, the show's producers say that his scene was fully animated, before being cut — by George Lucas himself — and "never making it out of the lab".
Everyone loves Knights of the Old Republic, and everyone loves Revan, but boy, seeing him in this forced death march of a toy commercial would have put a real bummer on my fond feelings for the guy.