This must surely be the first of all horse transformations! At the Dawn of Time, in the days before the Olympian Gods, the Titans ruled over the world. Chief amongst them was Cronus (or Kronos), and his wife, Rhea.
Cronus, like Zeus, his son, was not the most faithful of husbands, especially as a prophecy told that one of his ligitimate children would one day overthrow him. His roving eyes one day fell upon a beautiful Oceanide by the name of Philyra, who tended a spring on the slopes of Mount Pelion. According to the tales, Philyra was not at all reticent to return the Titan's favours and they were soon locked in carnal embrace.
Unfortunately, Rhea got wind that her husband was cheating on her and went looking for him. Sensing the approach of his wife, Cronus transformed himself into a stallion, and Philyra into a mare (Fillyra? Uh, maybe not).
Rhea was apparently completely duped by this deception, allowing the two lovers to complete what they'd started. To Philyra's great chagrin however, when the restored nymph gave birth nine months later, her child was human above the waist, but below that possessed the legs and body of a horse. Thus was the first centaur, Chiron, born. Chiron grew to greatness, but his mother never saw it: so dismayed was she at having given birth to what she percieved as a monster that she was overcome, and to ease her pain, Cronus transformed her into a Linden Tree.