Don Quixote was on foot with his horse unbridled and his lance leaning against a tree, and in short completely defenceless; he thought it best therefore to fold his arms and bow his head and reserve himself for a more favourable occasion and opportunity.
…cause of my dejection," returned Don Quixote, "is not that I have fallen into thy hands, O valiant Roque, whose fame is bounded by no limits on earth, but that my carelessness should have been so great that thy soldiers should have caught me unbridled, when it is my duty, according to the rule of knight-errantry which I profess, to be always on the alert and at all times my own sentinel; for let me tell thee, great Roque, had they found me on my horse, with my lance and shield, it…
There are no more uses of "unbridled" in the book.
Show samples from other sources
Wolves are also strict monogamists, and although I do not necessarily consider this an admirable trait, it does make the reputation for unbridled promiscuity which we have bestowed on the wolf somewhat hypocritical.
Farley Mowat -- Never Cry Wolf
It was attached to his saddle, which was cinched around his horse, which, when they stopped, could be subtly unbridled, just enough so that with a fierce tug from her end, the chain would rip the saddle off the beast, he’d tumble to the ground, and she would— She sensed Captain Westfall’s attention.