Big Octo is a mid boss found in the Nintendo 64 video game The Legend of Zelda: Ocarina of Time. He is found within the level known as Inside Jabu Jabu's Belly, and it's been assumed that Jabu Jabu, a large fish, actually engulfed the enemy, thus explaining its current location.
Big Octo is a large Octorok as its name implies. Link is required to use the newly found Boomerang in order to defeat him. Doing so will stun him, thus allowing you to go to the backside to attack its sole weak point. Interestingly, the boss can be defeated in one hit if the player chooses to use the Deku Stick on the boss's back.
In The Legend of Zelda: Majora's Mask, also for the Nintendo 64, there are three of these in the beginning of the game near the Deku Palace. They're much easier to kill this time around. Link can kill them by either running into them with his boat or shooting them with his bow.