Red Jellyfish move oddly, vibrating along the ground. They aren't very fast or unpredictable, but they dish out and take a lot of damage. They are one of the best Enemies at this point in the game to farm, though, especially if you use the Aqua Boots to strike safely from the water.
White Jellyfish are tied with Death Lords as the toughest Enemies in the overworld, at least in terms of hit points. They also dish out a lot of damage and drop a lot of gold, but they're still slow. They are a nice farming target once you have the Legendary Sword.