From the horse's mouth

It looks like the next character may be a guy and a girl on a moped. A moped is basically a tiny motorcycle with pedals. Or maybe it's more like a BMX with a tiny motor. Many of them were built in the 70s and 80s. They can look pretty funny carrying one person and even funnier carrying two. Jim mentioned this possibility in some comment.

New Vehicle Rumor: Akira Motorcycle!

I'm not the only one to discover this bug (or easter egg), but if you go to the level editor, select the character and manually type "5" in the input field to the right of "default character:", you will see the motorcycle from Akira! Although you can't test your level or play using this vehicle, it suggests this is a possible upcoming vehicle. When asked to comment on this, Jim Bonacci, creator of Happy Wheels, had this to say:

"EAT F***ING S***"

Seriously.

[UPDATE: With the release of the Moped Couple, this trick no longer works]