Keplian were first introduced in Three Against the Witch World.
Appearance: Keplian are pure black creatures that look like perfect horses. In earlier books they tend to have reddish eyes, signifying their allegiance to the Dark. After The Key of the Keplian, however, those serving the Light can have blue eyes.
Life Span: The lifespan of a Keplian is unknown. They might live as long as an Earth horse or their forming could have granted them a far longer lifespan.
Power: Keplian can seduce humans to ride them by appearing tame. They can run at supernaturally fast speeds and may be able to bind their riders to their backs so they can't escape. In The Warding of the Witch World the Keplians going on the southern mission were able to wall a Dark Gate by circling it, it is possible Keplians can do other things as well.
Other: Keplian can mind-send, though reading beyond the surface might be dangerous for an empowered individual - at least if they see the alien Keplian mind as evil. Eleeri proved in The Key of the Keplian that a human can read thought below the surface, but she did not have the prejudice that many others have. The Keplian were apparently created by those of the Light but one was created by (or chose to be a servant for) the Dark, and that Dark one tainted their entire race for generations until he was defeated.
Keplians cannot mate with normal horses without powerful magical aid. In fact, the Keplians turned to the Light in The Key of the Keplian consider the very idea disgusting. Monso, from Songsmith, seems to be the only hybrid of Keplian around. Whether Monso would have been affected by the events of Key of the Keplian is unknown. He does not appear in Warding, though the Keplian Eleeri helped do.