Bone Stallion

Bones. The Bone Stallion being just a skeleton of a horse allows for it to swim in water at the same speed as it walks.

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.

Mountable. Up to two players may mount the Bone Stallion in expense of half of their movement speed. Any melee attacks attempted against the riders on top of the Bone Stallion is made with disadvantage. 

Hooves. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit 2d6+4 bludgeoning damage.

Hooves. Melee Weapon Attack: +6 to hit, reach 5 ft., prone target. Hit 2d6+4 bludgeoning damage.