Demons of the Past: Book One of the Damewood Trilogy by Erin Durante
From the back cover:
MODERN CIVILIZATION HAS DISINTEGRATED
The technological advances of mankind have flourished beyond expectations and escaped the controlling hands of the human race. Cloned humans and genetically engineered creatures prowl the lands, pushing the earth over the edge to a third World War.
Four centuries later, the world has regressed to a feudalistic state, unaware of its advanced past. Now, the kingdoms live in fear of mutated "demons", of shadowy forests, and of frosts destroying their spring crops. It is a land where only the secret of society's past is known by a select few - and those few seem to have their own agenda on the upcoming successions of thrones.
Nadia, the eldest princess of Damewood, flees for her life after her castle is attacked by an underground cult bent on bringing society back to its "roots". Disguising herself as a knight, she joins a hunting party along with her best friend - a demon horse named Vestro with his own dark past - to put an end to the massacres and retrieve a stolen key that will unlock the cult's mysteries and the history of her people.