The dopefish is a creature from Commander Keen. If you want to know more about him, visit the Official Dopefish homepage. To make him appear, start a single player race and spin about one and a half circles to the left while riding the brake and keeping the lion that starts the race on your screen. If the lion disappears, or if you let off the brake, the Dopefish won’t show up. You can do it as many times as you like, as long as the lion does not leave the screen.
If you want to see the source code for the part of Wacky Wheels that loads the dopefish, see this page.