Why does a octopus have eight legs and begins with oct when a spider has eight legs but does not begin with oct?

There is no rule that says the name of a creature must be based on how many legs it has.

Spiders have been around a long time- I doubt that the first person who saw one counted its legs- would you?

A better question might be , "Why was the octopus so named? " -- I would guess, that being a sea creature, far removed from most people, it did not have a name when it was first seen by man, so somebody made up a name for it and chose to focus on its eight legs to choose its name.