In the the recent Unearthed Arcana they released Centaurs as a playable race. These creatures have an ability that says
Finally, a Medium or smaller creature can ride on your equine back if you allow it. In such a situation you continue to act independently, not as a controlled mount.
Centaurs are medium creatures. Thusly, my question is as follows:
So, a few minutes after I posted this question, I found some information from Jeremy Crawford which explained how it didn't work. One of his answers here says that it says "a Medium creature", which does not include creatures on top of other creatures:
Twitter user: Just so you know, the #DND hive mind has already decided that a medium centaur can ride a centaur riding a centaur ... (repeat) ... riding a centaur. The puzzle is solved. It's centaurs all the way down.
Crawford: If your DM decides that "a Medium creature" actually means "a tower of Medium creatures," that's an amazingly trippy, English-defying campaign you're playing. Live the dream!"
In response, someone else asked him to clarify whether centaurs could ride other centaurs; another tweet by Crawford explained that mounts must have suitable size and anatomy to ride it:
Twitter user: Doesn't that still mean that a centaur can ride another centaur?
Crawford: The rules on riding a mount specify that a mount must have a suitable size and anatomy for you to ride it (PH, 198). If the game makes an exception to part of that rule—about the creature's size, for instance—the other part of the rule still stands.
While I could personally see the argument that only one Medium creature would be "riding" the centaur, the second makes the centaur deathstack invalid.
No, the stack is not valid.
It comes down to a matter of weight. Whether or not we agree with Crawford regarding "suitable anatomy."
The rules on riding a mount specify that a mount must have a suitable size and anatomy for you to ride it (PH, 198). If the game makes an exception to part of that rule—about the creature's size, for instance—the other part of the rule still stands.
The matter is actually settled elsewhere in the rules. Carrying capacity, whether you use variant encumbrance or not, is still limited. So even if we disagree with Crawford, and think that centaurs have the right anatomy to carry other centaurs, we are still left with the issue of how much they can carry regardless of its shape.
Certainly, a particularly small centaur may be light enough for a particularly strong centaur to carry on their back. I would not be surprised if parents do this with their children regularly, and I would not be surprised to see one centaur carrying an injured comrade in this manor.
But player characters are typically adults, and we are talking about riding a mount, not being carried along due to injury. So a given, especially strong (str 20), PC centaur may be able to carry 600lbs.
Carrying Capacity. Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry, which is high enough that most characters don't usually have to worry about it.
Equine Build. You count as one size larger
when determining your carrying capacity and the weight you can push or dragSize and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag, or lift. For a Tiny creature, halve these weights.
A given PC centaur might possibly weigh less than that, depending on how "medium" PC centaurs really are. Therefore, it is possible for one centaur to act as a mount for another. It's possible to lift up to double your carrying capacity though, with limitations.
Push, Drag, or Lift. You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess of your carrying capacity, your speed drops to 5 feet.
So one additional centaur could be "lifted", but would reduce the speed of the bottom centaur to 5ft. Beyond that, the rule says "up to twice your carrying capacity." Which means that beyond that, beyond 1200lbs, it's impossible for the centaur to move the weight.
That is assuming that your DM even allows you to "push, drag, or lift" by carrying something on your back, which is highly questionable. Otherwise, you are stuck with the 600lb carrying capacity, and that's it.
So we can conclude that a centaur can carry another, if the DM decides that the anatomy is acceptable (personally, I don't see a problem). But they need to be a strong centaur, and/or the other one must be particularly small. Beyond a single rider, movement of the "stack" is reduced to 5ft if possible at all, and it's nearly impossible for there to be more than two centaurs being carried, unless they are children.
If you use variant encumbrance, even one centaur rider is going to cause encumbrance, probably heavy encumbrance - so movement will be reduced by 10-20 ft regardless, and you may:
...have disadvantage on ability checks, attack rolls, and saving throws that use Strength, Dexterity, or Constitution.
So just one rider that heavy is a massive liability, the "stack" would make everything worse. And don't forget, while the movement penalty only matters for the first centaur, all the others suffer from the disadvantage except for the one at the top. Meaning they are far less effective in combat than the same number of centaurs all moving independantly.
That said, this is Unearthed Arcana material and requires even more finishing than regular 5e material to be ready for play. Simple solutions include making Centaurs Large (I mean, horses are Large and regular Centaurs are large so...), generally prohibiting Towers of Rage by ruling that mounted creatures cannot themselves be mounted, or just ruling that centaurs can't mount stuff in general or always count as mounted and you can't mount more than one creature or similar.
But no, in the base game with the inclusion of this Unearthed Arcana there's nothing stopping this from working.
