Octosaks are enormous cephalopods that live in the oceans of Mongo.
In the comic strip, Flash Gordon and Prince Barin are accused by Ming the Merciless of being traitors, and sentenced to death. The High Priest says that according to the Holy Writ of Tao, the prisoners are allowed to fight for their lives. Flash and Barin are forced to fight on a narrow platform above a water-filled tank. Barin falls, and Flash jumps in after him, and battles an octosak lurking in the water. Flash breaks the monster's neck by grasping it "under its tearing beak". ("The Monsters of Mongo")
An octosak menaces the Hydrocycle bringing Flash Gordon and Dale Arden to Kala's city in Chapter 103: Captured by Shark Men of the 1936 Flash Gordon (serial) serial. Luckily, the octosak is distracted by a passing shark.
At the end of the chapter, King Kala traps Flash in a water-filled tank, where he's forced to battle an octosak. Princess Aura saves Flash in Chapter 104: Battling the Sea Beast, ordering Kala to drain the water from the tank.