I have always liked the idea of being able to climb onto the back of a collossal sized creature. Are there any rules or suggestions for how to implement this idea in Pathfinder. Would this be an exception to the grapple rules of size limitation, or would a climb check be more adequate? Any thoughts would be helpful thanks.

I've done this befoe, and it's strictly an on the fly type house rule so YMMMV

1) Initiate a grapple with a large or larger creature.
2) make a climb check, with a DC equal to, I guess, its CMD (though that seems a bit low.)
3) If successful, you are on its 'back'. you need to make a climb check every round as a move action to hang on. You are still limited to actions you could perform while grappling.
Changes to grappling in this manner.

a) As you are on it's back, you can no longer 'pin' the target.
b) You need both hands to climb properly. by taking a -5 to your round by round climb checks you may hang on with just the one hand (your initial grapple needs to be with both hands free though)
c) Your grappled target is flat footed to you. You have a +4 bonus to your AC for being in such an awkward position relative to the creature (but you are climbing, so you have no dex bonus) the creature retains it's dex bonus to defence against other combatants
d) If the creature want's to buck you off, he may do so as a move action. This forces another climb check with a peanlty equal to the creature's strength bonus.
e) If you fall off your target, you are prone. If you end the grapple, an acrobatics check (DC equal to the creatures CMD) allows you to land on your feet (If the creature was big enough you may take falling damage though)

Batts

I have always liked the idea of being able to climb onto the back of a collossal sized creature. Are there any rules or suggestions for how to implement this idea in Pathfinder. Would this be an exception to the grapple rules of size limitation, or would a climb check be more adequate? Any thoughts would be helpful thanks.

Thanks for the input. I was also wondering if the ride skill would be of any use or applicable in this situation?

Thanks for the input. I was also wondering if the ride skill would be of any use or applicable in this situation?

It's useable against foes size large or bigger, as long as you don't mind fighting with a 1-handed weapon in one hand and a piton or climbing spike in the other.

It effectively lets you enter the monsters space and stay with him while he moves. Although if you're hit for more than 10 damage you need to make a climb check to stay on, DC equal to the damage dealt.

Thanks again for the info; hopefully my gm will allow it :p