Octogon

Octogon acts as the Boss of the Mermaid's Cave in. It can spit rocks like its relatives, the Octoroks and does so when above the water. It bears a resemblance to the Big Octo that appears in.

Strategy
To defeat Octogon, Link must stand on the dry part of the room and wait for Octogon to turn and attempt to fire a rock, at which point he can attack it with his Sword or Seed Shooter, the sword is the best option as it will also stop the rock. After a while, Octogon will flee underwater, and Link must follow it using the Mermaid Suit and continue attacking it until it dies. Following its death, Link will receive the fifth Essence of Time, the Lonely Peak.