Hydrosoar

A rare, large mollusk specied creature from The Legend of Zelda: Link's Awakening, the monsters known as Cueball (there were only two documented cases of the creature) inhabited the water-filled labyrinth of Angler's Tunnel and the lava infested areas of Turtle Rock. The creatures used both liquids as a mean of propulsion around their domain. The only weak point on their body was their tender backside, which Link managed to exploit with the edge of his sword, killing both of the creatures.

In the Ocarina of Time, the Big Octo (Mini-Boss) is very similar in weakness, with it's backside being the weak point. However, there is an extra step of hitting it with the boomerang in order to make the spot open revealing a green attack point.