Final answer:
The field of vision of a horse not turning its head is vast and half-moon shaped, enabling it to detect predators across a large area.
Step-by-step explanation:
The field of vision of a horse, which is not turning its head, can be described as a huge half-moon shape, covering the area from the points of its head and tail, which are almost opposite each other, and potentially spanning a wide distance. This wide range of vision allows the horse to see nearly everything around it except directly in front or behind its body, thus lending to its ability as a prey animal to detect predators.