Fun fact, if I'm not mistaken many octopuses can not just change color, but also change shape to mimic their environment. In other words, an octopus trying to mimic a brick would not just take on the texture of the brick, but also the shape.
Although dictionaries will show words that significant parts of the population say are wrong, I'm still going to say octopuses is also correct because octopus is an English word and English usually pluralises like that
Octopusen is not common since octopus is too new to get that style of English plural
Of course significant parts of the population spell "than" with an e, or "then" with an a, and/or don't know the difference between it's and its