I was having the great "octopuses vs. octopi" debate with a scientist friend the other day, and decided to check the lexicon.
The only entry I could find relates the word to measurement, either of length or area, and the only available instance was Plato:
Socrates Do you observe, Meno, that I am not teaching the boy anything, but merely asking him each time? And now he supposes that he knows about the line required to make a figure of eight square feet; or do you not think he does?
Source: Plato, Meno 82e
The octopus was definitely known to the Greeks, as these vases demonstrate.
Q: Is it correct that there was no exclusive word for octopus, or cuttlefish in general, in Ancient Greek? Does the same hold for Latin?