Wansapanataym (Horse/Monster Transformations)

Wansapanataym is a Filipino fantasy show with multi-episode story arcs. In one  a character (or two) is cursed to become a Tikbalang, which wikipedia defines as "a creature of Philippine folklore said to lurk in the mountains and forests of the Philippines. It is generally described as a tall, bony humanoid creature with disproportionately long limbs...It has the head and feet of an animal, most commonly a horse."

I haven't yet seen the arc but there are clips on youtube.
In the first, "Boyong transforms into a demon horse" while talking to his girlfriend:

Next, "Atom drinks Boyong's magic milk, causing him to transform into a demon horse."

Boyong also has a short reversion scene. Apparently there are many other transformations on the show, but episodes are hard to find or download in decent quality.