The Shark is an enemy in the original Final Fantasy. They often appear in groups of two or alone. Due to their relatively high Attack power, they are the most dangerous enemies in the ocean when the Warriors of Light first set sail on their ship.
Use physical attacks, but if you are having trouble, use Thunder (or, if there are more than one of them, use Thundara). These are the strongest enemies in the game at the point where you first obtain the ship, so it's advised to head straight for Elfheim not worrying about fights until you purchase Thundara. After doing so, the Black Mage has a very quick and effective way of disposing of them.
Every version of the sprite is facing right, instead of left like in the original drawing.