gutter bunny
n. Mountain biker slang for a person who commutes to work on a bicycle.
No gutter bunnies allowed.
—“Look Out for That Rock…” (photo caption), Newsweek, June 16, 1997
gutter bunny n. a bike commuter.
—Tim Nelson, “How do you say crash? Bikers' lingo has countless ways,” Saint Paul Pioneer Press, August 08, 1996
1996 (earliest)
They're called "gutter bunnies."

They're the cyclists you see cranking along the snowbanks every morning in January, huffing home sweating bullets in August, and pedaling off to work, come rain, shine and everything in between. …

"It's a chance to show people not only that there's another way to get around and get to work," says Rich Moeller, of the state bicycle advisory board. "But it's also to show that there's a way that's better for them in terms of their health and getting some exercise."

Moeller, executive director of the Mesabi Family YMCA in Hibbing, is a gutter bunny himself.
—Tim Nelson, “Cyclers gear up for big day of fuel-saving,” Saint Paul Pioneer Press, May 09, 1996