The second horse is oriented in the opposite direction: it appears to be grazing toward the right, with all four legs touching the ground and its tail pointing left. Because the two animals face away from each other, their individual body parts can appear to belong to a single, impossible creature.
Once the direction of each horse is identified, the image reorganizes itself. One animal runs along the left side of the composition; the other occupies the right. The overlap that seemed so confusing becomes a deliberate arrangement.
The answer:
