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 reeks a bit too much of furry for the typical MG fan.
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.