Chauncy and Suzy Elliott at the Twin Rivers CCI* in 2008. © Candace Funk
Elliott had dismounted before Chauncy collapsed.
Organisers said there was a full paramedic and veterinary team present within moments after the collapse.
Chauncy was taken to LaSalle Veterinary Hospital in Kalispell for a necropsy, which revealed a cardiac hemorrhage.
Chauncy, who was owned by Suzy's husband, Jonathan Elliott, was by Desert Wine.
The Elliotts, who run Aspen Farm in Washington, described Chauncy as one of their "young talent".