Okie dokie, fair enough.

magic or cold iron it is.
On to more interesting topics…
Since we’ve decided that the horse is the natural form, how about this:
Change Shape (Su): A chevall’s natural form is that of a horse. In this form, a chevall may be of any color (though each individual cannot change its color), and is about the size of a (light war horse?).
A chevall also assume the form of a centaur; a chevall in its centaur form always assumes the same appearance and traits, much like a lycanthrope would. A chevall’s centaur form has the upper torso of a human, as with normal centaurs, and its lower body is the same as in its horse form, though somewhat smaller than the average centaur’s. the centaur form’s ears are pointed like an elf’s.
A chevall remains in one form until it chooses to assume a new one. A change in form cannot be dispelled, nor does the chevall revert to its natural form when killed. A true seeing spell, however, reveals its natural form if it is in centaur form.
In either of their two forms, chevalls 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.
Once per day, a chevall can magically summon 1d3 war horses which arrive in 1d4 rounds.
(slightly different in GAZ1: “They may speak with horses, in either horse or centaur form, and can magically summon 1d6 war horses (which arrive in 1d4 rounds) once per day.”
not sure how much, if any, of this part from the Mystara MC we can or should use:
Chevall Speed Table
Movement - Horse form - Centaur form
Walk - 12 - 9
Trot - 24 - 18
Canter - 36 - 27
Gallop - 48 - 36
A chevall can carry no more than 260 pounds and still travel at its full speed. A can travel at half speed while carrying up to 390 pounds, and can move at one-third speed while carrying up to 520 pounds.
As noted in the Player's Handbook (Chapter 14), in a day of travel over good terrain, a creature can travel a number of miles equal to twice its normal movement rate (a trot); that is, a chevall in horse form can cover 48 miles. In dire circumstances, a chevall can push itself to a canter or gallop. A canter can be safely maintained for two hours, or a gallop for one hour, but then the chevall must walk for an hour before increasing its speed again. A chevall will not gallop if loaded with enough material to reduce its normal movement rate by half: nor will it canter or gallop while carrying a load which will reduce its normal movement rate to one-third normal.