Gun Runner has claimed one last win over Arrogate, being named the United States Horse of the Year.

The soon-to-be-retired five-year-old won four Grade One races during 2018 including the Breeders' Cup Classic.

Arrogate, Gun Runner's top rival for much of 2017 as well as the leading money-winner in North American thoroughbred racing history, and World Approval were the other Horse of the Year finalists.

"He stepped up when called upon and just met the challenge and, you know, how do you say, got it done," trainer Steve Asmussen said.

"He's truly a special horse mentally and physically. And we've been on a tremendous run with him."

The run ends on Saturday (Sunday AEDT) when Gun Runner races for the final time as the favourite in the $16 million Pegasus World Cup Invitational.

"Just to be in his presence, to be around him, to have the privilege to train him as well as run him, you know, it's a dream come true," Asmussen said.

Gun Runner also picked up the Eclipse Award as the top Older Dirt Horse in 2017.

Chad Brown won the Eclipse Award as the sport's top trainer for the second consecutive year while Jose Ortiz, the leading money winner last year, ended the four-year run of Javier Castellano as the best rider.

Brown paid tribute to the other finalists, Asmussen and Bob Baffert.

"I'm always learning from people," Brown said. "Those are two guys I'm always learning from."

Juddmonte Farms was picked as top owner for the fourth time, and Clearsky Farms got its first Eclipse as top breeder.