Indifference to human capacity, I think, assumes that there is a thinking mind that is at liberty to chose what there is to pay attention to. As such, horses are not indifferent to human activity but rather respond to our approaches in instinctual flight-or-fight responses, something coming from a survival instinct deeply enscribed in their genetic code. Freedom in any general consideration is based on the assumption that individuals are capable of making decisions and enacting choices within the communities they live in; I doubt horses have anything resembling the capacity to make decisions based on a reasoned sizing up of options.

I do like the poem, though, and I don't think Richardson is anthropomorphizing the animals. Beautiful as these animals are, the qualities we give them beyond their species nature are more from our imaginations than anything innate in their stately frames. Horses wandered out of the pen because the gate was left open and horses are, after all, no less likely to prefer group behavior over individual endeavor.