From 1d4chan

Chevalls are frankly one of the most bizarre therianthrope species in the entire cosmology of Dungeons & Dragons. Native to the world of Mystara, Chevalls are centaurs with the ability to transform into normal horses.

...Yes. You read that right. Well, technically they claim to be horses given sapience and the power to transform into a centaur form by a Mystaran Immortal that wished to protect horses from mistreatment by humanoids.

Technically part of the Fey races, like the centaurs they resemble, Chevalls appear as strong, healthy horses (around the size of a light war horse), with the normal colors; when they take on their centaur form, they are typically smaller than average, and their humanoid torso looks more like an elf than a human. In either form, a chevall can talk to and understand horses, using sounds which, to human ears, are nothing more than neighs and whinnies. Using this whinnying language, a chevall can command any horse, wild or domesticated, to do its bidding. A paladin’s warhorse, and other unusual mounts with average Intelligence or higher, are immune. In centaur form, a chevall can speak Common, the language of centaurs, and woodland sylvan tongue. Much like the lythari, another fey therianthrope, chevalls have the same resistance to non-magical or non-silvered weapons as a true therianthrope.

Chevalls strive to ensure the well-being of all horses. Once native to the plains, they now appear anywhere that wild or captive horses exist. They often go about in horse form, checking on the welfare of horses in the sewice of humans, demihumans, and humanoids. If a chevall finds a horse that is unhappy with its lot (because of maltreatment or neglect), the chevall will not rest until it has freed the animal.

A chevall travels alone or in groups of up to three. If three chevalls are encountered, there is a 50% chance the group is a mated pair and a foal (which has half the Hit Dice and inflicts half the damage of adult specimens).

Foraging sustains chevalls as they travel. They favor vegetables and grains, and may (in horse form) gain nourishment from grazing (although they consider grass a very bland food, and prefer tasty oats and barley). While they are omnivorous by nature, most chevalls adhere to a vegetarian diet. This may stem from moral conviction or sheer habit.

With the exception of horses, other animals recoil from chevalls due to their therianthropic aura. They have a deep, abiding hatred of wolves, and especially of werewolves, and eagerly battle the latter wherever they encounter them.

Chevalls debuted in Creature Crucible: Night Howlers for BECMI, and were updated to Advanced Dungeons & Dragons 2nd edition in the Monstrous Compendium Appendix Mystara. They have not appeared since. And despite what you'd think, they've never been presented as playable.