From Nookipedia, the Animal Crossing wiki
Donating to the museum
In Wild World
"I think Octopus, and I think fried octo-bits, eh wot? That said, I have to say my all-time favorite has to be octo-potpie. Hoo hoo!"
In City Folk
Blathers's comment is as follows:
"Ah, yes, hoo, the octopus. Did you know the blood of the octopus is actually colored blue? With its eight legs and predilection for spewing ink...you'd think people would be repulsed by it. And yet, perhaps due to its fetching shape, the octopus remains one of the most loved sea creatures!"
The Octopus appears in the back tank, staying in the rock crevice towards the right. If the Octopus is placed in a home, it appears in a medium square tank with rocks so it can live in the crevice.
In New Leaf
The Octopus is now classed as a deep sea creature rather than a fish, meaning that it is no longer caught by using a fishing rod; rather, the player must dive underwater once they have obtained a wet suit. After being donated, it can be found hiding in a pot in the large pool in the first room of the Ocean exhibit. The exhibit has this to say about the Octopus:
"Many people think octopuses have eight legs, but those are not legs. Their tentacles are actually arms. They have a strange anatomy, with their head located between their body and arms. The flexible arms are perfect for catching all kinds of crab, shrimp, and shellfish, especially in reefs. When attacked by predators, octopuses emit a black ink that spreads through the water. This greatly reduces visibility and allows them to escape, making them highly skilled at disappearing."
Octopus rubescens generally grows to a mantle length of 8-10 cm, and arm length of 30-40 cm. Adult weight is generally 100-150 grams, however occasionally animals up to 400 grams in weight have been observed. Like all octopuses, O. rubescens can change its color and texture, making its appearance highly variable. Color can vary from a deep brick red, to brown, to white, or mottled mixtures of the three. The octopus from Animal Crossing series is the East Pacific Red Octopus.