Rider at the Gate — C.J. Cherryh

A forgotten colony lost on a planet of wild beauty and unimaginable danger. A people held together by a renegade band of outcast heroes, called to their destinies by alien dreams. A world of Riders and Nighthorses, their minds linked together on the knife-edge of an abyss--for even a single Nighthorse can annihilate the human race.

And now one Nighthorse has gone insane...