Hydrosoar

A large mollusk-like creature from Link's Awakening, the monster known as Cue Ball 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. The only weak point on their body is their tender backside, which Link must 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 Cue Ball as he passes beneath Link in the longer straightaways and then strike him.

Trivia

 * Cue Ball is known as or "Hiploop Hover" in Japan, connecting it to both Helmasaurs who are also known as "Hiploop" in Japan and share the monster's armored head and weak behind, and Hover, a water-skating enemy that is also known as a Water Tektite in Link's Awakening. Both enemies appear in the Angler's Tunnel along with Cue Ball.
 * Fans also call it a "Water Cephalopod", a play off of Fire Cephalopod, because both are fought in a room filled with liquids. Both creatures used liquids as a means of propulsion around their domain, and they also bear somewhat a resemblance to real cephalopods.
 * In Ocarina of Time, Big Octo is very similar in weakness, with its backside being the weak point. However, there is the extra step of hitting it with the boomerang in order to make the spot open revealing a green attack point.