There are three type horses in Cheju horse racing.
The first type is Cheju horse, They are natural monuments. The horses must be confirmed their DNA by KRA. KRA has been trying to establish bloodline between them. The second type is Cheju-bred horse, its heights must be in 1m 25cm ~ 1m 33cm. KRA checks the heights every 6 months, and kicks out horses over 1m 33cm. The third type is Domestic ponies, its heights must be below 1m 25cm. KRA also checks the heights every 6 months, if horses grow up over 1m 25cm, they should run in Cheju-bred horse race.
At the beginning of Cheju horse racing, there's not enough 'Cheju horses' to race. For a racing, KRA classfied race horses just by heights, so there were 'Thoroughbreds x Cheju horses' among them. Nowadays, KRA gives incentives to trainers, owners, breeders of Cheju horses.