Here are some possible scenarios where a horse might be described as "milling":
* Nervousness or boredom: A horse might mill around in its stall if it's bored or anxious.
* Seeking something: A horse might mill around in a field, looking for something to eat or a place to roll.
* A specific training technique: A trainer might ask a horse to "mill" around an obstacle as part of a training exercise.
It's important to note: While "milled" is not a common term to describe a horse's action, it's a perfectly valid word to use in these specific contexts.