Warlock 10's Fighting Fantasy adventure "Rogue Mage" by Graham Davies has this paragraph.
It's a Man - It's an Octopus - It's an Octopus Man - It's a Man-Octopus!
I had assumed that was just a two-tentacled octopus thing with a human face on it, but perhaps it's supposed to be a human torso and legs with an two-tentacled octopus with a human face on it. I think I prefer the former. Intelligent? Maybe, but I prefer the idea of the face being an idiot, drooling one which is more horrific.
Rogue Mage is a solo for one Level 1 Fighter which is probably the why the attacks only do d3 damage each so I'd beef the damage up to 1-6/1-6. What weapons? Probably sharp bits of coral, crustacean shell or rusty metal ripped off sunken ships.
B/X Stats follow
Also have this Gary Harrod picture from PROTEUS 6 - The Fortress of Kruglach of a Giant Cycloptic Octopus aka MOAT MONSTER in PROTEUS-speak.
(Treat as B/X Giant Octopus - it's probably crapper with missile weapons than a regular two-eyed Giant Octopus but in honesty this is highly unlikely to ever be important).
Lastly the third right arm of a male octopus is usually his penis so for shits and giggles any octopus encountered by the PCs is male on a 4-6 on d6. This means that any blow from his tentacle should be accompanied by a roll of d8 - on a 8 the PC has been struck by a Giant Octopus Cock which has no real in-game effect save for the embarrassment factor which will doubled should be the penis blow prove fatal.