The Magic Seahorse is an enemy in The Sea. It shoots magical blue balls at you. They don't move forward toward you, so you need to either have a Monkey Wizard Staff, an Octopus King's Crown with Jaspers, Fireball Spell, Acid Rain Spell or Black demons. The erase magic spell would work well here but be sure to have enough health so that you don't get killed. There would be one seahorse moving up and down at a regular rate while another would follow your movement up and down.
Strategy
Using one of the weapons mentioned above, move into the seahorse's path and wait for the fireballs or magic balls to kill them. Quickly move out of the way as a seahorse's magic blue ball is about to hit you. Continue until all of the seahorses are killed.
If you only have access to either kind of fireballs, it is also possible to place yourself right in front of a stationary seahorse, just below the blue balls it shoots: you are then high enough to shoot fireballs at it without getting hit.