Post-traumatic stress disorder (PTSD) and other psychiatric conditions are common among military veterans. Effective conventional treatments exist; however, many veterans do not engage in treatment and among those that do, many have an incomplete response. Thus, equine-assisted services are being increasingly used as complementary interventions for this population. However, due to a lack of rigorous studies, the field is in the early stage of scientific development. Research is needed to both establish benefits and determine mechanisms of action. Despite the lack of rigorous research, the existing literature is promising. For example, investigators have reported decreased depression, anxiety, craving for substances, and PTSD symptoms. Additionally, improvements in psychological flexibility and affect have been reported. Further, there appears to be a low risk of adverse outcomes. Mechanism of action studies are needed as little is currently known about mechanisms of change underlying benefits. Potential mechanisms include healing associated with the horse–human relationship, biophilia, enhanced psychological flexibility, problem-solving through metaphor, emotional mirroring, and heart rate synchronization. The path to move the field forward requires rigorous randomized controlled trials of interventions that are developed to be manualized and replicated and which include both psychological and physiological outcome measures.