At least according to the press release from the Official Website of the Beijing 2008 Olympic Games' Equestrian Page....
USA announces Eventing roster for Beijing Olympics
(BEIJING, July 17) -- The US Equestrian Federation has named its Eventing roster for the Beijing Olympic Games and is waiting for approval from the US Olympic Committee, Teamusa.org reported on July 15.
Five rider-horse combinations, and four replacements, have been selected for the Eventing team:
Phillip Dutton, aged 44, riding Connaught, a 15-year-old female Irish Sport Horse
The press release was from the Official Website of the Beijing 2008 Olympic Games. I "Googled" Olympic Games Equestrian Hong Kong, and got the website. I didn't check to see if the other disciplines were riding "females" as well.