A cave

As she came down from her latest orgasm Vivia noticed the landscape had changed slightly. The horse had carried her through a valley among taller hills and cliffs, slowing to navigate the winding terrain. Before long she found herself in the shadows of towering bluffs, the smooth slopes giving way to sheer, uneven foothills.

As the horse came to a breathless stop to graze, she managed to catch her own breath, her body offered some reprieve from the constant motion, yet still bound to the beast tightly. Vivia spent some time using her remaining strength to struggle against the bonds, but found little success in anything beyond rubbing herself against the saddle some more. When she stopped to take in her surroundings, she noticed the yawning mouth of a cave not far off.

In fact, she saw a few different caves when she was able to twist her head around to look. As the horse lazily grazed around a small patch of grass, she was carried to a view of the nearest cave just a couple-dozen paces away, where after a moment of study she noticed a pair of eyes watching her from the shadows within.

She couldn't make out more than a faint refraction indicating they were there, but someone—or something—was definitely peering out over the crest of a boulder inside the cave, looking right at her. She didn't know whether or not it would be wise to try and call out for help through the bit in her mouth...