Giant octopi are not at all uncommon in the world of Godzilla. Although they have an odd tendency to come on land more often than normal octopi. There have been at least two. The first was seen on Mondo Island, and attacked the villagers there, until King Kong appeared, fought the octopus, and drove it away.
Sometime later, a second giant octopus appeared inland in Japan, and attacked Frankenstein following his battle against Baragon. The huge mollusk easily overpowered the weakened irradiated giant human, and dragged him to a watery grave in a nearby lake. The following year, the same octopus attacked a Japanese fishing vessel during a storm, seeking to devour the crew. Its attack was thwarted by the arrival of Frankenstein's genetic "son," Gaira, who defeated and drove the octopus away, only to then eat the ship's crew himself.