The octopus might have been a spoof on the way Groening drew hands:
- A human should have 5 fingers, and is drawn with only 4.
- An octopus should have 8 tentacles, and -using Groening's human hands as a humorous rule demanding a limb reduction by one- therefore will be drawn with only 7 tentacles.
(Of course, it might have been just a mistake on a "What the heck? No one will notice wether this bugger has seven or eight arms anyway, and we are on a budget and therefore will not spend time on fixing that mistake. The seven armed octopus is good enough for our viewers...." basis