In 1999, Stacy began exploring scientific methods to promote rapid healing and recovery in the large animals on which she operated. With the help of her friends scientist Sue Bates (Polymer) and fellow veterinarian Joe Frost (Doctor Frost), Stacy invented a bioplastic bandage to wrap around the surgical incision after the operation to cause rapid cellular regeneration. Before she could test the bandage, she cut herself badly on a broken test tube containing horse's blood. Unable to receive medical attention in time to help her, Stacy wrapped her cut arm in the experimental wrapping before she blacked out. When she came to, she discovered she was as strong and fast as a horse, and could even alter her form into any horse-related animal. Taking a cue from her friends the Omegas, Stacy adopted the identity of Equestria to battle crime and injustice.

The combination of the experimental bioplastic and horse's genetic material gave Equestria horse-related megapowers that run along evolutionary lines. She is as strong as a Clydesdale and can run as fast as a racehorse (about 40 mph). She can also alter her form into any sort of equine animal, from the prehistoric eohippus to a horse, pony, zebra, mule, or donkey. It is possible she can also take the form of mythological horse-related creatures, such as the pegasus (winged horse), unicorn (horned horse), nightmare (bat-winged horse with fiery hooves and poison fangs), hippocampus (horse with a fish tail), and more.