The chicken wasn't wearing any pants.

Jeremy Powers couldn't help but notice. Powers is one of the elite cyclocross racers in the world, the U.S. national champion in 2012, and a couple years back, he was in the middle of a race—he thinks it was Portland, Ore.,—pedaling his bike, pushing that red needle of exhaustion, when he became aware of an exuberant, feather-costumed fan running alongside him.

"A...