A wild pony died Thursday in a rare accident after taking part in the traditional swim from Assateague Island to Chincoteague Island on the Atlantic Coast of Virginia.

“Butterfly Kisses tragically died this afternoon after a freak accident in the pen at the carnival grounds,” the Chincoteague Volunteer Fire Co. said on its Facebook site.

It said the pony was being chased by another. “As she was running, she slipped, fell hard to the ground and slid into the fence,” the account said.

It said Butterfly Kisses broke her neck and had no chance of survival.

According to traditional practice, wild ponies from a herd on Assateague Island make an escorted swim once a year at the end of July across the Assateague Channel. Thousands of spectators from all over the world come to the coastal area for the event, which was in its 93rd year, the fire company said.

After a rest, the ponies are assembled at the carnival grounds, where an auction of foals takes place the day after the swim.

According to the fire company, the auction helps keep the Assateague wild pony herd from over expansion. It also raises money for the fire company. Some of the money raised is used, according to the fire company, for veterinary care for the ponies throughout the year.

Butterfly Kisses, the pony that died, had a foal, the fire company said. The foal was bought by a local family, who took it home, the fire company said, and began bottle-feeding it.

By tradition, the adult ponies swim back to Assateague on the day after the auction, to live for another year in the wild.