How can a bond with a horse help a human heal? 

Moving a human from trauma to healing requires the restructuring of emotional response. Both primary and secondary process emotions (conscious and pre-conscious) must be engaged and soothed (regulation of the sensitive limbic region of the brain). This must come in the form of new experiences. The experience of limbic neuroplasticity (the laying down of new neural pathways and responses) was deeply and beautifully expressed by a client in the following: 

"What I noticed more than ever today was how all of the horses responded to me. They came close with softness. I did have a story with some alarm come up around the big black, but then I took a belly breath and allowed him to come close. I touched him on the side of his face and acknowledged his presence by looking into his eye. I used to go into such deep states of disconnect that I could not even look anyone in the eyes and my body felt like it was made out of concrete.

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“Horses and Their Ability to Heal” by Gloria Lybecker, originally published in Natural Awakenings Inland Northwest. 

"The horse has evolved over millions of years as a prey animal. Their nervous systems are wired to react quickly to known or perceived threats. The physical structure of a horse's brain, with its enlarged amygdala, closely resembles humans who have experienced trauma. This gives them the ability to engage easily with those who struggle with hyper-vigilance and reactivity, providing the "neuroception of safety" (safety felt by the subconscious) and intuitive understanding of who is trustworthy and who is not. 

It is often said that horses see with their ears. This is true in that their visual to mental focus can be observed by where they point their ears. Yet every movement of the horse is communication: a twitch of its ear, a swish of its tail, the snake-like movements it can make with its neck. 

Horses also have huge hearts, averaging about...