Rider control. If rider is nearby, heavy warhorse has advantage on saving throws against charmed and frightened condition.
Heavy Combat Trained. Heavy warhorse is proficient with light, medium and heavy armor. Also a rider on heavy warhorse gets a advantage on all checks to maintain control or not dropping from saddle.
Possible traits
Any heavy warhorse can have three of those traits.
Trampling Charge. If the horse moves at least 20 feet straight toward a creature and then hits it with a hooves attack on the same turn, that target must succeed on a DC 14 Strength saving throw or be knocked prone. If the target is prone, the horse can make another attack with its hooves against it as a bonus action.
Mobile Charge. After galloping for 20 feet or more, this creature does not provoke attacks of opportunity when moving in or out of range of any enemy.
Harassment. If charging a unit smaller than itself, upon impact they must make a strength check, DC15, to avoid being knocked prone for one round. Anyone caught flat footed or fleeing is automatically knocked down.
Endurance. A heavy warhorse make Constitution saving throw against exhausted with advantage.
Run. A heavy warhorse can make a Dash as bonus action.