Carny was a android, a automaton with human form. A short and fat human form. He was also an unofficial member of the robotic Extremists and the ruler of Wackyland, an amusement park created by Mitch Wacky, (a Walt Disney parody) of Angor and the last vestige of the civilization than once existed on the planet. It was never clarified if Carny ever was also the robotic version of a previously existent human Carny or if always was a robot designed by Mitch Wacky.
Carny had taken hostage the Silver Sorceress and the JLE after the Extremist attacked earth and sent them to the devastated Angor. But as a good robotic host, Carny was just delighted to have new visitors again, visitors to whom entertain. But after he revealed the true nature of the park and after the rejection of the leaguers to be entertained, Carny made the automatons attack the earthmen. However, the JLE just struck back and went after Carny. Terrified, Carny did the most wackiest thing he could: he destroyed the park and himself in the process. But that destruction woke up Mitch Wacky from his cryogenical suspension.
The head of Carny survived the explosions and was reactivated later by Wacky himself. There, Carny told him and the leaguers what happened after the death of the humans, how the automatons upgraded themselves and how they built heroes and villains so realistic than the villains believe themselves the real ones and destroyed the heroes, left Carny in charge of the park. This information was relevant to defeat the Extremists later on earth.
Countdown: Earth 8
Another version of Carny made a brief cameo as an inhabitant of Earth-8. He was the only one who opposed to some scheme to Lord Havok and was killed almost in the act. He looked more like a circus clown in this version.
Convergence: Batman & Robin.
Bring back in Convergence, a very fleshed Carny appeared as representative of Angor against the pre-Flashpoint Gotham City, defended by the team of Batman, Robin (Damian Wayne) Red Hood and Scarlet. Carny was leading the New Extremists: Death Angel, Brute and Meanstreak and using lethal gadgets to attack the caped crusaders, but their teamwork overcame the powers of Carny and their extremists team. It was presumed than he and their teammates were returned to their proper world/universes/timelines. Also, it wasn't clarified if this Carny was the original one or the robotic version.
As his colleagues Extremists, Carny is assumed to be a DC "homage" to some Marvel villain. By his relation to an amusement park is associated to x-villain Arcade.