Final answer:
The horse and pony have the same relative speed if they are moving across a field at the same constant speed, irrespective of their size difference.
Step-by-step explanation:
If a horse and a pony walk across a field at the same constant speed, and it is given that the pony is half the size of the horse, we can conclude that their relative speeds are the same. This is because speed is defined as distance divided by time (v=d/t), and as stated in both the question and principles of physics, if they are moving at the same constant speed, their relative speeds are equal regardless of their size.