MetaFilter: “You’d chase them under the tank, back and forth, like you were chasing a cat“ links to Deep Intellect –
Inside the mind of the octopus by Sy Montgomery.

There are some other interesting links in the comments, a mimic octopus alternately morphing into a flatfish, several sea snakes, and a lionfish and the oldie-but-goldie Where’s the octopus? (longer version than the one you may have seen years and years ago).

There’s more octopus goodness in this older thread, for example this anecdote about a pet octopus.