[stephenb (Stephen)]

Welcome James, I think what your probably thinking of is rather than an ancient, formalised constellation originating from a specific culture, you are perhaps referring to local asterism which is a pattern of stars which is not one of the 88 formal constellations.

For example, if you look at the attached images of the night sky at around 8:30pm (now), looking South. The first image depicts the formal constellations (joined by the lines). You can see Crux (the Southern Cross) on its side to the right. Underneath it, closer to the horizon is the constellation of Carina.

Look at the second image with the constellation lines removed. Four of the stars in the constellation of Carina make a prominent "cross". Many astronomers call this "The False Cross". It is not an official constellation, but an asterism, an informal pattern which is decernable in the sky. There are lots of these (you can make up your own if it helps to identify with the sky).

I suspect that the story you have heard of the "Octopus" is simply an asterism that someone has created out of simply looking at the sky long enough. Kind of like watching clouds in the daytime sky. Stare at them long enough and you will start to see shapes.

Many years ago a good friend and very respected amateur astronomer pointed out an asterism he called the "Wild Duck". It spanned 1/3 of the sky and represented a duck, its beak and outstretched wings. Unfortunately he has passed away, and to this day I cannot locate it in the southern sky, but I'll locate it one day.