You could patent your game design, if that's possible. This is exactly what patents were created for. But you yourself are probably churning out original (instead of copycat) little games and can't afford to patent all of them.
But you can copyright the artistic aspects of your game, like you can have the copyright to MONOPOLY and trademark the logo.
If you could patent gameplay there would be only 1 pong, which is a pretty basic idea. There are probably hundreds of pongs though.