The young stallion named Bashful tried to jump a 1.8m enclosure. He nearly made it, but his back legs became caught in thick wires in the pen.
The two-year-old's front legs struck and injured an elderly man watching the auction, who was taken to hospital. The Bismarck Tribune named the victim as William Friedt.
Bashful ultimately sold for $35 and was off to a new home.
The sale was marked by strong prices for the horses taken from the North Dakota park.
The top price at the sale in Dickinson was $US1425 was for a three-month-old foal.