So I was running some theoretical build work and it suddenly dawned on me that there's nothing stopping you from mounting a creature that would then mount a creature. So my thought process goes as follows
Step 1; Play a Summoner, get to high level
Step 2; Make your Eidolon a huge quadruped mount with lots of arms, arm it with 2 Lances (cause on a mount you can use a lance one handed), tons of claw attacks, and both the Grab and Rake special ability.
Step 3; Get a Roc, and mount your Eidolon onto it (It has max ranks in ride and handle animal, so it can probably ride better than you can)
Step 4; Murdertown???
Your pouncing, hasted Eidolon should get 7 Lance attacks on the charge, each dealing about 3d6+150 (Cause your Eidolon took Spirited Charge and maxed it's strength of course) and its to hit can be so high even its tertiary attacks could hit a Balor on a 1.
Then you would have your creature makes 6 claw attacks or so, each one fully power attacked (Cause obviously you're full power attacking), and EVERY time you hit (which is always) you get to make a grapple check which if you win (which is always cause you're huge and have a strength of 50) you generate two ADDITIONAL "Rake" attacks which are virtually identical to your claw attacks.
Then you rend.
All in all the damage a mounted (mounted) Eidolon can deal looks like it's in the thousands. And then of course YOU get to go.
Anyone ever think about doing this before?