The caption says the first horses to arrive and here’s what I found about the first horses to arrive
[I]HONG KONG (AP) — The first 32 of the more than 200 horses taking part in next month’s Olympic equestrian events arrived in Hong Kong on Saturday to begin acclimatization to the city’s tropical heat.
The horses were transported to the stables at the equestrian venues after arriving and undergoing preliminary quarantine tests at Hong Kong International Airport.
“All traveled very well and the disembarkation process went very smoothly,” veterinary surgeon Christopher Riggs said.
The horses came from Belgium, Denmark, Korea, the Netherlands, Switzerland, Canada, Germany and the United States. A total of 229 horses is expected by the end of this month.[/I]
So, I would guess you could narrow it down to those countries.