Geldings are unbreedable horses. This is because geldings are castrated horses or other equines (e.g. donkeys and mules) This exterminates hormonally driven behavior, It allows the male horse to be calmer and better behaved. This can make their skills quite high as well as their genetic potential.
You can tell if a horse if a gelding or not by looking in the description, on gender. However, if you are buying a horse instead of the usual gender symbols by the horse's picture there will be a blue circle, this indicates that the horse is a gelding.
Geldings cannot reproduce when given a Zeus' Lightning Bolt.