What is a close relative to a octopus?

Answer 1

This depends on how far back you want to go.

The order for octopus is called octopoda. This falls in the class cephalopod, which includes squid, cuttlefish, and nautilus but out of prehistory you've also got the ammonites.

This is in the phylum mollusca which includes snails and slugs.

Lophotrochozoa is the last meaningful groups for layfolk (past that you hit everything with left right symmetry.) At this point we're looking at flatworms and rotifers but because you're probably not familiar with those I'll talk a bit about the body parts lophotrochozoans have. They either have a horseshoe shaped band of tentacles or their larva have two bands of cillia that they use to move around in the water.

The modern day species around no doubt differ from the actual common ancestors these all share but the simpler things further back along the chain are probably very very similar and up near the octopus you should be able to imagine something that is partway between all the close relatives but maybe less advanced than any of them.

Answer 2

One of the ancestors of the octopus is a cephalopod. That was the first one

Answer 3

a swimming squid

Answer 4

The squid and the cuttlefish.