So what is it?

Pawing, snorting, breath like fog,
Called by Indians, the "Big Dog".
They run wild, free as the sky,
Their spirits seem to go and fly.


Riderless, they flee the sound,
Of the wild coyote hound,
They fly away from the ropes,
Just to say, they ain't dopes.


To have to be under man's care,
Would be to be caught in a snare,
They'd run, free as the sky,
Flying--higher than high.

~Storyteller

So, what is it?