It is Greek, not Latin.
'octo', meaning 8, plus 'pous' meaning foot.
The plural of 'pous' would be 'podes', because it is third declension. However, in ancient Greek, I believe this word was usually treated as undeclinable, since it doesn't make sense to pluralize the second element.
Sorry, the word 'octopus' is entirely Latin in its etymology. 'Octo' = Latin for 'eight', 'pus', derivation from 'pes' = Latin nominative singular for 'foot'.
You're smart. Seriously. I wish I knew that kind of stuff but my eyes glaze over when I try to learn it.