Is this really true? Wikipedia says:
“Stallions were often used as destriers due to their natural aggression. However, there may have been some use of mares by European warriors, and mares, who were quieter and less likely to call out and betray their position to the enemy, were the preferred war horse of the Moors, who invaded various parts of Southern Europe from 700 AD through the 15th century. Geldings were used in war by the Teutonic Knights, and known as “monk horses” (German Mönchpferde or Mönchhengste). One advantage was if captured by the enemy, they could not be used to improve local bloodstock, thus maintaining the Knights’ superiority in horseflesh”