Cueball

From Zelda Wiki.org

Cueball
Game(s)	Link's Awakening
Dungeon(s)	Angler's Tunnel
Weakness(es)	Sword

A large mollusk-like creature from Link's Awakening, the monster known as Cueball inhabited the water-filled labyrinth of Angler's Tunnel. It is identical in appearance and behavior to the Fire Cephalopod found in a lava-filled area of Turtle Rock. Fans also call it a Water Cephalopod, because both are fought in a room filled with liquids. Both creatures used 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. It's possible for Link to equip the Roc's Feather and leap over Cueball as he passes beneath Link in the longer straightaways and then strike him.

In Ocarina of Time, Big Octo 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.