Why October is 10 and octopus is 8?

Octopus has eight legs which is why it is "8". The latin prefix "octo" means eight.

October was originally the eight month when the last six months of the calender was basically numbers (Quin, sex, sept, octo, novem, & decem).

To the point: Mars was the 1st month but January and Febuary were added later on moving all the months up two numbers.