Hippocampus

Horse in front, fish behind, and it moves through water the way a good horse moves over turf. They run in herds along the coastal shelf and are curious about boats without being frightened of them. A hippocampus will follow a vessel for two days for no reason anyone has established, and it will let a swimmer take hold of its mane exactly once, to see what happens.

Trample in the Surf: bludgeoning, melee 5 ft, +4 to hit, 2d6+3, and it fights far better in water than out. Bow Wave: 20 ft, save Dex Quick, fail save roll 2d6+2 bludgeoning as the herd turns together and the water goes with them, cooldown 3 rounds.

Of the Water: in open water it cannot be caught, cornered or outrun, and it attacks at Advantage. Beached it is helpless and knows it, and will not come far enough in to risk that. Takes a Rider: a creature that treats one well may be carried, and a mounted hippocampus is faster than any ship afloat. This is the entire reason to meet one.