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The general question of this post is: "Have I misunderstood a rule, or does this actually work RAW?"
This isn't so much something I want to actually build as much as something I can't stop thinking about and giggling. I think at 19th level you could have a reasonably okay build where your eidolon rides you. I think it is technically legal. So very, very technically.
You need to have the 8th level evolutionist ability to change base forms at any level increase, and of course we'll be making heavy use of Aspect and Greater Aspect. I think there are a number of technically legal ways to go about this, but this one I think involves the least gerrymandering.
With Aspect at 10th you give yourself the limbs(legs) evolution.
At 18th you have a large quadruped eidolon, counter-intuitively so that you can give yourself the Mount evolution. The rules of Aspect state that the eidolon must be able to take the evolution, and this makes that possible for Medium Summoners. [Mount requires quad base form and to be one size category larger than the summoner.] You could argue that a Summoner cannot take it as he does not have the quadruped base form, but I can't find any rule saying 'Humanoid summoners are considered as having the biped base form for the purposes of selecting evolutions through his Aspect class feature.' The text of the RAW state that eidolons must have the quadruped base form, no such caveat exists for the Summoner himself. Besides, we have four legs already, so there. We may as well also take Improved Natural Armor or something.
At 19th we use the remaining 4 evolution points of our Greater Aspect to give ourselves large size, while simultaneously taking it away from our eidolon. The restriction on the Mount evolution's selection seems only to apply when it is selected. Unlike feats, there is no caveat stating that one loses the benefits of the evolution if prerequisites are removed.
We also change the eidolon's base form to biped, and likely give it the Skilled(Ride) evolution.
This may sound mostly useless, but oddly I think you could make a pretty absurdly powerful lancer eidolon using this. You'd have to be building the eidolon's feats toward it for its entire career, but you could just be using your Summon Monster ability if you liked until you can finally accomplish your character's lifelong dream of being a horse.