While it is understood that a centaur could be used as a mount under certain conditions, such is in this question, I am asking whether or not a Centaur is capable of using a mount for itself. Is there an contraption or creature that is capably of serving a Centaur as its mount? Are there any mechanics that allow Centaurs to use more mundane mounts?
For Reference, the Centaur race in 5th Edition is located within Guildmaster's Guide to Ravnica, pages 15 - 16.
The rules on mounted combat state (emphasis mine):
A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules.
Now, finding a creature that's at least one size larger is pretty easy. The playable centaur race is size Medium, so any Large creature satisfies the first requirement. You might even find a willing one. But I think you would be hard-pressed to find a willing Large creature that has an "appropriate anatomy" to be ridden by a centaur.
Rules designer Jeremy Crawford unofficially addressed a semi-related question in a May 2018 tweet (though the question was about the UA version of the centaur race and whether they could be stacked):
Doesn't that still mean that a centaur can ride another centaur?
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.
But who or what determines suitable anatomy? Cuz the way I see it is that a centaur would have suitable anatomy for another centaur BECAUSE they're the same shape. Have you stacked up cups in a cabinet or something? They're still the same size but they stack very effectively.
The DM decides.
As referenced in the tweet, the Equine Build trait in the UA version of the trait had made an exception (as the tweet mentions) to the size requirement of the mount rules: "Finally, a Medium or smaller creature can ride on your equine back if you allow it." However, as Crawford states, even if the size requirement is overridden, the "appropriate anatomy" requirement is not.
This exception to the size requirement was removed from the trait in the published version of the centaur that appears in the Guildmasters' Guide to Ravnica (p. 16). Thus, no exception is made at all to the rules on mounts. Even if you find a creature of a suitable size (Large, or larger), it must still have a suitable anatomy to be mounted.
