Hydrosoar

Cue Ball, also known by the name Water Celphalopod, is a large octopus-like creature from Link's Awakening that first appeared as a mini-boss in the water-filled labyrinth of Angler's Tunnel. The front of its body is heavily armored and cannot be harmed. The only weak point on its body is their tender backside, which Link must exploit with the edge of his sword. This is easiest done by waiting for it to pass by and striking from the side, but it is also possible for him to chase it down with the Pegasus Boots. Upon each hit, Cue Ball will spin and change direction. 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. Once defeated, Link will gain access to the Flippers.

Cue Ball later returns in Turtle Rock, known there alternatively as Fire Celphalopod. While it behaves the same as its aquatic variation, it now swims around a pool of lava, making the battle considerably more challenging for Link. In this dungeon, Cue Ball guards the room where the Nightmare Key is hidden.

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.
 * 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.