This is an exercise in ridiculousness, but I find it quite fun to consider – hence powergaming.
Let's consider three characters: a gnomish cavalier (small) with his wolf mount (medium), half-orc druid (medium) and druid's companion, a gorilla (large).
I remember the rules saying that the rider can mount a creature one size category larger than himself. Thus, our druid is able to mount his gorilla (PFSRD even lists gorillas under Ride skill), but not the wolf.
But under the same premise nothing stops the gnome from mounting a half-orc, provided the orc is willing. This has tremendous advantages as we can assume the half-orc is combat trained and obviously has more than a few combat uses. We can imagine a gnome piggybacking on a charging half-orc and it seems that in optimising Cavalier this notion is fairly accepted.
However, I am wondering, since half-orc can mount a gorilla and gnome can mount the half-orc, can the gnome mount the half-orc while he's mounted on his gorilla? Is there anything in the rules that prevent this kind of cheese, apart from Rule Zero?
Particular usage:
I wanted to have a set of characters ranging from as small as I can get (currently with a Small race – any Tiny playable characters out there?) up to having a Huge mount (say, a Mammoth with a Mammoth Rider PRC) giving me an effective stack of 4 (Small, Medium, Large, Huge) characters, though not necessarily PCs, say, animal companions, summons, familiars, just regular mounts. After gathering a number of creatures willing to participate, I'd make sure the top one is a Barbarian with Ferocious Mount and Greater Ferocious Mount. This causes the barbarian's mount to use his rage and all of his rage powers that do not require an action to activate and are constant when raging. It just so happens that Ferocious Mount and Greater Ferocious Mount are such powers, so in my understanding all the applicable rage powers would propagate down the Stack'O'Mounts fueling this ungodly tower. Now the focus would be to enhance each mount's attacks, be those natural attacks or otherwise. However, if Stack'O'Mounts could possibly charge, Beast Totem theme has some very powerful choices. If not, then maybe getting a wereboar-kin to be on top could enhance each mount's gore attacks, that could be a lot of Con damage.
Best Answer
Towering Mountain of Rage
Right, you want to to propagate Rage effects throughout a tower of creatures. That's... actually kind of interesting. Beast Totem chain grants Pounce and 2 Claw attacks, substantially improving the combat prospects of all creatures in the tower.
Well, the applicable rules are contained here, Pathfinder's Mounted Combat Rules on PFSRD.
Looking them over, we have some... interesting interactions. For a start, as your Half-Orc mount is not a 'combat-trained mount', you have to make a move action to control him in battle as he becomes frightened. Yes, the condition. No, I am not making this up.
But hopefully we could waive that somewhat ill-considered clause (what about an intelligent animal? ugh) and assume a half-orc with player class levels is 'trained for combat riding'. So, you're looking at a DC 5 check, probably with a -5 since a half-orc is not 'suited to riding', to keep the use of both your hands and 'control him with your knees'.
I'm struggling to contain my giggles as I write this.
A slightly more difficult attack is to 'Fight With A Combat Trained Mount', a DC 10 Ride Check, with again that pesky -5 for being ill-suited as a Mount. With Ride as a Class Skill, though, it's probably pretty easy to make this check, especially with an Exotic Military Saddle.
Note that EACH rider would have to make this check, so 3/4 of the stack if you're going Mammoth->Gorilla->Half-Orc->Gnome. I'm not sure if Gorillas get Ride as a class skill. This is particularly problematic as if the Gorilla gets unseated from the Mammoth... well, actually, both the Half-Orc and the Gnome would get to roll to 'Stay in saddle', actually, which is only a DC 5 check. Presumably the Gorilla could attempt to 'Fast Mount' in his next turn, thus returning the stack bonuses to the Mammoth.
I can't find any rules in the Mounted Combat section or the Ride skill to disallow this.
I also think it would be awesome, and utterly approve of the mobile 'Tower of Rage'. Note that the Entire Stack would act on the Initiative of the gnome ("Your mount acts on your initiative count as you direct it. You move at its speed, but the mount uses its action to move.")
I assume this means that the stack acts on the gnome's initiative, but moves at the Mammoth's speed (and if it charges, they all count as charging). Note, I am unsure if a Small Character with 10' of reach from a spear could actually reach the ground if he was on the back of a half-orc on the back of a gorilla on the back of mammoth. I think he couldn't, actually. Ergo, he should probably multiclass to bard and do some form of 'rage music', because, why not.
There is a catch, however; Ferocious Mount mentions that you must spend an extra round of rage to spread the rage to your mount, which the mounts can't do as they have no rage class ability. Greater Ferocious Mount makes no mention of this, but Rage powers don't activate if the wielder is not in a Rage, SO while you can make a Tower of Mounts, you cannot make all those mounts rage unless each mount in the stack is also a Barbarian.
It's worth noting that if each mount is a Barbarian, they will pass on any continuous Rage powers during the Rage to any Barbarians lower in the stack, potentially making the lowest Barbarian have many many many Rage Powers.
But yes. Unfortunately as is it doesn't work unless all but the lowest 'mount' in the stack are all Barbarians with enough Rage rounds to make this worth doing.
Awakened Cat Barbarian riding a Gnome Barbarian riding an Orc Barbarian/Warchanter with war Drums riding a Minotaur Barbarian riding a Huge-Ass War-Mammoth is essentially 90% of the point of playing Dungeons and Dragons.
In 3.5e they would all be wielding monks, as monks are a manufactured weapon.
Probably it would end up looking a little bit like this;
YOUR LIGHTNING IS ALL I NEED