Scylla
Scylla is a monster from Classical mythology, a former nymph who was transformed into a hideous creature by a jealous rival, envious of her beauty and the fact she had won the romantic favor of a god. In pop mythology, she looks like a beautiful woman from the waist up, but with tentacles instead of legs and with six wolves/wolf heads on extendable necks ringing her waist, which she sends out over a considerable distance to snatch up sailors from passing ships so she could eat them. In the actual mythological tales, she's much more monstrous: one story describes her as having twelve "dangling feet" and six "hideous heads" with triple-rows of teeth on long necks, another as having the body of a serpent or a fish with six wolves sprouting from her waist, one says she has "twelve tentacles and a cat's tail", and one describes her as being a human from the waist up and six wolves from the waist down.
Amongst monstergirl fans, "scylla" has become a popular name to use for monstergirls who mix human girl with octopus, most commonly in the style of a mermaid; human from the waist up, huge octopus, squid or cuttlefish from the waist down(such that she has eight tentacled inplace of legs, which give the impressinon of a dress when hanging right). The term is contentious, since Scylla the mythical monster isn't exactly an octopus girl, and there is a growing preference for the term "cecaelia" instead. Actual MG depictions of the mythological Scylla are much rarer, since the whole "she has dogs growing out of her waist" thing sound more like a John Carpenter monster than anything someone of sound mind would be attracted to.
The Monster Girl Encyclopedia is a perfect example of the "scylla is an octopus-merfolk monstergirl" thing. The Kraken (here a hugely busty mini-giantess squid-woman) and the Mindflayer are both considered part of the scylla's family tree.
Scylla (alongside her often-forgotten mythological partner, Charybdis) features as a species of monster in the Pathfinder setting. They appear as beautiful women from the waist up, with wolf heads in a ring around their waist and giant octopus tentacles in lieu of legs. Classic "octomaids" also feature in the same setting, called Cecaelia.