The Paladin spells Find Steed (Player’s Handbook pg.240) and Find Greater Steed (Xanathar’s Guide to Everything pg.156) list the following creatures that are valid targets to be summoned as mounts:
- a warhorse
- a pony
- a camel
- an elk
- or a mastiff
Find Greater Steed
- a griffon
- a pegasus
- a peryton
- a dire wolf
- a rhinoceros
- or a saber-toothed tiger
However, of these options, the Pony, Mastiff, and Peryton creatures are classified as "Medium" creatures. And per the Mounted Combat rules found in the Player’s Handbook (pg.198), a valid mount must be a size bigger than its rider:
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.
The description for Find Steed stipulates that the summoned creature serves as a mount for the creature that summons them:
Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit.
Is this a case of Specific Beats General, where a medium-sized creature very well could ride on these magically-conjured medium steeds, or is such a creature still bound to obey the restrictions placed on them by the Mounted Combat rules?
A related question about Perytons being summoned by Find Greater Steed touches on this issue, but is scoped around questioning whether the statistics of a summoned Peryton are different from a normal Peryton. As a result, I don’t think it’s a duplicate of this question: What happens if I summon a Peryton with Find Greater Steed?