D&D Basic Monsters: Warhorse

Although a war horse technically is any horse trained not to panic or bolt in combat, regardless of size. In a medieval setting, however, it makes sense that warhorses are larger specimens capable of carrying a plate mailed knight while wearing cumbersome armour (barding) and being able to trample lesser foes underfoot. 

The most famous of the medieval war horses were the destrier, used in jousts and tournaments as well as on the battlefield. They were known for their great size and strength, though they were also extremely expensive, and most knights would use what was known as a courser, a slightly smaller, but strong and fast type of horse. In large battles, the knight would usually ride to the site of the battle on a different horse (a palfrey) to ensure that the warhorse was as fresh and rested as possible before charging.