nic'Epona are a race of magical equines native to the Outer Planes in the Great Wheel. Reputed to be daughters of Epona, the Celtic goddess of horses, nic'Epona resemble brightly colored (they can change hues as they wish, though most settle on a favorite color) mares with exceptionally high intelligence (average Int of 15-16, in AD&D), the ability to speak humanoid languages, and a number of other inherent magical powers. Primarily, they can run on literally any surface (which allows them to mimic flight by running on air), and they can Plane Shift at will. For this reason, many have tried to take advantage of them as steeds, but the highly intelligent nic'Epona naturally take considerable offense to this. As one of their magical powers is a total immunity to any effect that normally commands magical obedience from animals, they like to feign obedience and then dump their would-be rider in a hostile planar environment.
nic'Epona are an all-female species that reproduces by mating with stallions from other equine races, including magical ones such as the pegasus, unicorn, and even nightmare races. Fillies are more nic'Epona, whilst colts are members of their father's species.
They debuted in the Planescape setting and have been largely forgotten ever since. They were updated to 3e in the Planar Handbook, where they were renamed Ur'Epona.
In Dungeons & Dragons 5th Edition, nic'Epona resurfaced in the fan-made Forgotten Realms subsetting of Anchorome, in the splatbook "The Herd of Epona". Whilst largely preserving the lore of their Planescape counterparts, there are unique elements to these nic'Epona, in particular their membership in the Chic'Epona - the Great Herd, a mystical alliance of magical equines that includes asperii, comet steeds, equar, sleipning, Diomedan mares and other equine creatures alongside unicorns, pegasi, mundane horses and crossbreeds beyond imagining.
In the author's own words, this reimagination was inspired at least a little by My Little Pony.