Rocky is light brown in color and is the same size as a snail. Also, it looks like a scaled down version of Patrick's rock.
Role in series
Patrick enters Rocky in a snail race competing against Gary and Squidward's snail, Snellie. Somehow, it actually wins the snail race despite not showing any signs of sentience.
In "Buried in Time," Patrick wants to donate Rocky to the time capsule. In this episode, however, Rocky is lumpy and gray, possibly meaning that he somehow got older.
In the beginning of "Patrick's Staycation," when SpongeBob says "Ooh, a shadow puppet show," Patrick says "SpongeBob, please," while holding Rocky.
In "The Endless Summer," when Patrick says to Mr. Krabs that he and the people of Bikini Bottom are moving to colder climate, he is seen holding a stick with Rocky strapped with a rope to the end of the stick.
In "The Great Snail Race," Rocky does not move, but at the end he is last seen on the finish line.
Perhaps the Mayor put Rocky at the finish line so that the race would have a winner.
Rocky could have won by Patrick helping him along the track without stepping on the track himself, but that begs the question of why Patrick wasn't disqualified for setting foot on the track.
According to Patrick, Rocky has nerves of steel. Patrick continually mentions this, saying, "I know, I know, he's got nerves of steel."
In "Buried in Time," Rocky looks lumpier and is gray.