You have a limited understanding of the variety of Latin declension. Greek words were commonly used in Latin, with their forms adapted to a greater or lesser extent.
"Octopus" means "eight-footer". To see how to decline it, we have only to look at the word "tripus", meaning "three-footer". Its plural was "tripodes". Based on this:
In the singular
In the plural
- Nom/voc: octopodes
- Acc: octopodas (octopodes)
- Gen: octopodum
- Dat: octopodibus
- Abl: octopodibus
The forms in parentheses are more Latinised ones. To Latinise it even more, one could change the po to pe, but that would be going over the top.
These are the scholarly forms. It's not inconceivable that the noun could become misconstrued as second-declension in spoken Latin (this happened with polypus), but to deliberately do so would be to put a mistake in the Romans' mouths. Whether we think of it as Latin or Greek, its plural is "octopodes". For English usage, however, the anglicised "octopuses" is usually better.