Climbing onto a creature makes that creature a mount
A creature needn't be friendly nor mount-like for a creature to attempt to use it as a mount. Because mounted combat is given the short shrift by the Player's Handbook, I've very reluctantly relied on parts 1, 2, 3, 4, and 5 of Skip Williams' Rules of the Game columns "All about Mounts" for parts of this answer. While these columns are often derided for their inaccuracies, without them, this process of playing a rider astride a hostile mount is impossible and, more broadly, mounted combat is nearly opaque and nearly unplayable and requires a raft of house rules to make it playable. So rather than dismissing or ignoring these columns, as long as they're expanding and not contradicting the core rules, I think they're safe to use for this purpose. If nothing else, they provide some insight into the game's expectations according to one of the game's lead designers and the author of its Monster Manual. Anyway, the turn of a creature that wants to use a hostile creature as a mount looks like this:
- The potential rider takes a move action that puts the potential rider adjacent to the potential mount. The potential mount must be at least one size category bigger than the potential rider.
The potential rider makes a Ride skill check to fast mount (DC 20) and suffers a −5 penalty on this check if the creature is "ill-suited as a mount," suffers a −5 penalty on this check if attempting to fast mount a creature lacking a saddle, and suffers his armor's armor check penalty on the check (see PH 80 for all these modifiers). Success means that the rider takes a free action to become the mount's rider. Failure means that the rider takes a move action to become the mount's rider.
- While it goes unstated, a hostile potential mount should be able to make an attack of opportunity against a potential rider when the potential rider enters the hostile potential mount's space, much like when a Tiny or littler creature provokes an attack of opportunity when it enters a Small or bigger foe's space to make a melee attack or like when a creature provokes an attack of opportunity when it enters the foe's space when making a bull rush or overrun attempt.
- So far as I can tell, a rider that takes a free action to become the mount's rider can then take a move action to do something other than take a normal move, like draw a weapon or open an adjacent door, but can't take a move action that involves moving any distance, the rider's movement now dependent upon the mount's movement. Further, if the rider hasn't yet, the rider could instead take a standard action while astride the hostile mount, like making standard attack against the mount.
Things to remember
- A creature threatens into its own space unless armed only with a reach weapon, and the rider shares all of the mount's space. Further, a mount apparently suffers no penalties on attack rolls against a rider and vice versa. (The rider doesn't even get the +1 bonus on attack rolls for higher ground as the mount isn't "a creature smaller than [the] mount that is on foot" (PH 157), but, instead, obviously, the same size as itself.)
- Although they share the mount's space, neither rider nor mount suffer any of the effects of, for example, being involved in a grapple. Both can make attacks of opportunity normally, for example, and ranged attacks launched at one don't have a 50% chance of striking either.
Riding a totally hostile mount that's trying to eat the rider isn't detailed in the core rules nor in the columns, but under Unofficial Optional Rules (in a column with a lot of unofficial optional rules but this one is actually called that) on Riding along with an Aggressive Mount in "All about Mounts, Part 4" there's the following:
If your mount isn't trained for war but chooses to fight with you aboard [as opposed to simply fleeing the engagement], you can just give the mount its head. You and your mount make separate initiative rolls [or act on different initiative counts]. Because there are some full-round actions you cannot perform while your mount moves (see ["All about Mounts"] Parts Three and Four), you must delay until after your mount's turn to use such actions if your mount's turn in the initiative order comes before your own turn comes. Even then your mount's movements could keep you from performing your intended action.
When it's finally your turn to act, you must succeed on a DC 10 Ride check to adjust your actions to fit your mount's uncontrolled movements. The check is a move action for you, but it does not provoke attacks of opportunity. (You're looking after yourself, not your mount.) If you fail, your mount's movements keep you from taking any actions this round. If you succeed, you can use a standard action. Just remember that you're allowing your mount to take you where it will, so your options might prove limited.
Thus the typical rider will usually be stuck with taking only a standard action (or a move action instead) while astride that hostile mount if the rider can take any actions at all. The hostile mount, on the other hoof, can just stay where it's at (or take a 5-ft. step) and full attack the crap out of the rider, usually making this a really bad deal for the rider.
As an aside, the alternative to giving the mount its head (the link's to the term's definition; it's okay—it's work-safe) is for the rider to convince the mount via wild empathy or pleasant conversation to obey the rider. Good luck with that, Mr. I'ma-gonna-ride-this-here-behir.
A hostile mount can force an uninvited rider off of it by successfully tripping the rider (see Tripping a Mounted Defender (Rules Compendium 145)). Alternatively, the mount seems to be able to able to easily attempt a bull rush against its rider—the two creatures already share the mount's space, so it's possible the mount won't even provoke an attack of opportunity, but a failed bull rush attempt renders the mount prone as the space it's "returning to" is occupied by the rider. (Then things get complicated for the mount—see Sharing Spaces (RC 95).) A Huge or bigger mount may have to beat a Medium rider's check by more than 5 to shove a rider out of the mount's own space.
The tactical feat Giantbane (Complete Warrior 111) and its tactical maneuver climb aboard mechanizes this tactic in an entirely different way and was published 2 years before these columns. Also, the style feat Hammer and Piton (Dungeonscape 45-6) mechanizes this tactic in a third and different way and was published 2 years after these columns appeared. The method described above, however, has the advantage of being available to any creature without a creature needing to light a feat on fire to do something many kind of think any adventurer—if he really wants to—should be able to do anyway.
And even though it certainly should have, the Dragon #336 Silicon Sorcery column covering the video game Shadow of the Colossus (88-90) sadly doesn't include rules for running around atop the creature, although it includes rules for gaining purchase on one: the primordial colossus's extraordinary ability impassive says, "Creatures daring to approach a primordial colossus can make a DC 20 Climb check. If succssful, the creature is capable of a grabbing hold of the colossus and riding upon it." If the colossus is moving when this is attempted and the attempt fails, the creature that failed is dealt 4d8+19 points of damage. That's the extent of information about doing that, though.