Yuan Jing, an archaeologist with the Chinese Academy of Social Sciences, studied the more than 600 terracotta horses within the tomb of Qinshihuang, the first emperor of the Qin Dynasty, who ruled from 221 BC to 207 BC.
He noted that all the 520 horses that pulled chariots had penises but no testicles.
However, some of the 116 cavalry horses were found to have testicles.
Yuan said his findings gave some indication of how horses may have been handled by humans.
The tomb, located on the outskits of Xi'an, the capital of Shaanxi province, was unearthed in 1974 by peasants digging for water.
Today, it is listed as a world heritage site by UNESCO and is a major tourist attraction.
Researchers believe the terracotta army, which includes archers and infantrymen, was to help Qinshihuang rule in the afterlife.
There is evidence of pig castration dating back 3000 years, with descriptions of the practice written on shells.
However, researchers have yet to unearth actual evidence of horse castration on ancient horse skeletons.