Types of Horses

These rules are based on Robert Harrison's article "Let the horse buyer beware" from Dragon Magazine

Size Base Price Hit Dice Base Move* Damage/attack Carrying weight (pounds)
Light 50 2 24" 2d4+2 Hoof/1d4+2 Bite 300/500
Medium 125 2+2 18" 2d6+3 Hoof/1d6+3 Bite 400/650
Heavy 200 3+3 15" 2d6+4 Hoof/1d8+3 Bite 500/700
Quality Rating Price Modifier Hit Die Type Move Modifier*
Poor -50% d10-2 (min 1) +(2d4-6)"
Average 0 d10 +(2d4-5)"
Good +100% d10+1 (max 10) +(2d4-4)"
Excellent +200% d8+2 +(2d4-3)"
Superb +400% d6+4 +(2d4-2)"

*Take movement in inches and multiply by 2.5 to get movement in feet per turn. The warhorse in the PHB should have a 40 foot move.
Higher quality horses may be faster and have a higher minimum hit points. A character who makes a DC10 Animal Handling roll can get an idea of a horse's quality. DC15 roll lets them compare a group of horses to pick a better horse.

Training Type Cost (GP) Panic/Spell Failure
Riding +25 90%
Basic +100 40%
Skirmish +200 10%
Combat +500 3%

So a medium warhorse (125 GP) of Good quality (+100% of price or +125 GP) with Skirmish training (+200 GP) costs 450gp. If it was Superb quality (+400% or +500GP) with Combat training (+500GP), it would cost 1,125 GP.

Panic is the chance the horse will try to bolt when there is combat (smell of blood, screams, monsters or it takes a wound) and the chance that a spellcaster trying to cast from horseback will be jostled and the spell ruined. A horse that panics will move at full speed in a random direction away from the threat. Each turn the rider can make an Animal Handling roll to get the horse under control (Difficulty is based on the horse's training; DC20 for Riding, DC 18 for Basic, DC 15 for Skirmish and DC 10 for Combat). Druids or characters with the Mounted Combatant feat roll with advantage.

A horse with basic training will obey the rider's commands to charge and trample a man sized target (that uses the rider's Attack action). A skirmish trained horse will attack a creature that attacks it as well. A combat trained horse will attack targets in front of it by biting if possible or a hoof attack. If a combat trained horse's rider is unhorsed, make a panic roll. On a failure the horse will attack targets as it moves away. On a success, the horse will stand by its rider and attack creatures that approach.