You can actually stand on bushes for a moment before sinking. Navigate up and around to the nest, then use the bushes to jump into the nest. On the edge is a dead horse in a giant nest. Navigate around the cliffs until you can reach the upper edges of the tall cliffside. To find the horse armor, travel into the deep woods east of Ledetchko. It also looks nice aesthetically on the horse. The armor is for the head of the horse, and increases defense when equipped. There is currently only one known location it can be obtained. Horse armor is extremely rare in the game.