Millions! BS. There are reasons we have these legends. THOUSAANDS of years ago this thing existed. People saw it, probably hunted it to extinction. The legend was born.
Or maybe they are still down there.
Lurking.
Waiting.
Plotting.
Or, also possible, someone found a fossil beak and compared it to the size of a beak from a regular octopus and went "Yikes! Here be BIG monsters."
Ether way the octopus is a freaky deeky beast and they creep me out.
I with you, that they could still have still been around when humans first went out on the seas in boats.