Most horses, especially racehorses, birthdays are celebrate on January 1st in the Northern Hemisphere and August 1st in the Southern Hemisphere.
A horse born towards the end of the year or close to August would be considered a year old on those dates.
This sounds a bit unfair when it comes to racehorses as some of them can be much younger than their rivals and not as well prepared. Racehorses can compete at 2 years old.
A horse’s gestation period is approximately 11 months or 340 days. Breeders of horses that will enter competitions often try to breed their mares close to the January 1st and August 1st dates. Since they will drop their foals soon after these dates, their horses will have more experience training for racing and other competitive sports.
The January 1st and August 1st birthdays are really for horses in competitions. You can choose to celebrate your horse’s birthday then or on the actual date of birth.