About the plurals: The word "octopus" is Greek in origin, not Latin, and would be properly pluralised "octopodes". People who work around them at the Steinhart Aquarium in San Francisco tend to say "octopusses". "Octopi" is right out.
Have you seen one? They are amazing to watch. They change color quickly: from white to red to greenish brown in the time it takes you to say "Wow!". Not only color, but texture: they can change from soap-smooth to the texture of a turkish towel as quickly as they change color.
I have often found them in the tidepools just south of San Francisco. Their camouflage is so good that you can look and look where your friend is pointing, there, in the water, by the rock and the coraline algae, and not see it. Wow.