Rocky the "rock" is light brown in color and is the same size as a snail (Gary, Snellie, and Larry Luciano) Also, it looks like a scaled down version of Patrick's rock.
Patrick enters Rocky in a snail race competing against Gary and Squidward's snail, Snellie. Somehow, it actually wins the snail race. It somewhat resembles a snail, but with a rock for a shell in the episode "The Great Snail Race."
In "Buried in Time," Patrick wants to donate Rocky to the Time Capsule. In this episode, however, Rocky is lumpy and gray, maybe meaning that he somehow got older.
In "The Great Snail Race," Rocky does not move, but at the end he is last seen on the finish line.