Octopus are not squid. Squid don't live long, and never get overly tough (chewy yes, ohmigawd it's like rubber! Not so much.) Octopi, on the other hand, can live for years and get to be so tough they are inedible unless you know how to prepare them.
The best way to do this is to braise them in their own juices over low heat for a very long time. This concentrates their flavor and renders them dense and delicious, not rubbery and off-putting.
Even though there are several steps to this process, it's well worth it -- octopus is relatively inexpensive if you buy it at Asian or Mexican markets frozen, like this 5-pounder above.