Cephalopods are a group of highly intelligent ocean-dwelling invertabrates with multiple muscular arms. The group includes squid, octopi, cuttlefish, and similar creatures.
An octopus is an intelligent, ocean-dwelling invertebrate. With multiple muscular, ridged arms, a capacity to squirt ink clouds, an ability to change colors, a large, curious brain, and a fierce beak, octopi are masterful hunters in their domains. They in turn are hunted by larger fish, sharks, and many civilized races.
A monstrous octopus is an immense breed, some ten feet in diameter, often with "teeth" on the suckers of its arms.
Squid dwell in the open ocean more so than near coral beds like octopi. Squid tend to be aggressive and inquisitive, and aren't above mobbing a creature they feel might feed them.
A monstrous is an immense specimen. The largest squid have been known to eat whales.