Equestrian, art of horse-riding and trust between human and horse. It’s actual naming and recognition as a sport was during the summer Olympics of 1900 (depending on your country it can be debated if this date was the very start or end of a century) but the actual training and riding of horses researchers can confirm has been around since the BC eras divided in its earliest between War and Trade.
However the ability to work with horses has varieties from person to person. To some Horses are but mindless beasts that deserve nothing but to be broken into respectless slaves, some view horses with dread and fear for how one cannot easily pensive their acts, those that see the riding and racing as the possibilities yet respect a hidden intelligence and emotions, even those so amazed at their positive nature they secretly wished to be them...
...and by ‘Wished’, well that’s to an until now secretive magical research that alongside a number of other magical aspects that were exposed exactly halfway through the 21st century that gave a way to help with that and as a bonus help those with the fear overcome them.
These are these Halters. Ones a somewhat common sight on horses for guiding horses and rarely (but ill-advised) racing them. But these don’t seal or erase any minds let alone a horse’s but instead transform a human into one when worn upon their head, the process adapting them to the Halter. Reverting simply requires (Often with help) Removing the Halter. Some more advanced versions (Adult only mind) also allow one to switch genders while a horse.
One of the few issues known is that what breed of horse one becomes is tricky to determine most of the time, only that one’s ethnic background is a factor with horses usually being the equivalent of the ethnic origin place though rare exception exists. Also your horse fitness is exactly its counterpart to human pre-transformation though training often adjusts between sides if possible with certain people often choosing to train as horses for running and jumping endurance.
Then again like many things these Halters are prone to being abused. Perhaps mostly over how one can either impregnate or get impregnated while as a horse with a number of criminal courts now using a forced transformation as a punishment for sex crimes. What’s more there is a dispute over what happens to children or foals whatever you will during said pregnancies. Like being pregnant already converts the child in the womb into a foal but not foals into children if impregnated as a horse...
...not only that but giving birth as a horse while having been impregnated as a human results in the foal becoming the natural form (born hoses cannot be made into humans of course). What really brings this into question is some incident about a woman that went missing for a year when found insisting that this man who abducted her (Who was apparently the scion of a powerful and wealthy family that in all but officially owned the town they lived in) had done so after she fell pregnant by him and he forced her transformation into a mare until the birth to cover it up. He of course denies this and claims she was a madwoman who tried getting human children via horse pregnancy and not helping matters was the only labs willing to run the tests were local...
...last heard the woman is being treated in a specialized medical ward.
But despite all this these Halters have opened up vast new racing events, training and summer camps focused on equestrian, alternatives for continuing bloodlines for famous racers when normal mates are inaccessible (both forced and volunteer versions) and even rarely some even find ways to have personal ownership to these Halters: