Josh Hill 705's page

Currently there are no rules for riding an eidolon. In the final APG, there will be an evolution that allows your eidolon to be ridden as a mount.

For OP, I'd likely also stipulate (once that evolution is legal for play) that you have to choose quadruped in order to ride your eidolon.

An eidolon is not an animal companion. It's a called creature. Ranger, druid, paladin, etc. all have animal companions--the summoner does not. So if you were a Summoner 1/Druid 1 you would have an eidolon and a druid animal companion.

As for mounted combat rules, if you were to ride your eidolon (again, once it becomes legal for play) you would be subject to the mounted combat rules in their entirety.

Thanks for the clarafication. I appreciate the answer.

I was wondering how an eidolon would work as a mount in OP?

Could you use the biped base and give it two extra legs or do you have to use the quadruped?

Also if I take another class with an animal companion do the levels stack for my effective Eidolon level since it is considered an animal companion?

Last Question is the mounted combat rules. Can both do actions in the same round if they don't move? Since in the rules it states the mount uses its action to move.