Peter Shaffer is not actually trying to make a point about the power of horses and their influence on humans, but rather of the power of human emotions and their influences. The play is ultimately a play about psychology and especially explores the power of spiritual belief, especially the consequences of suppressing spiritual belief. As the play progresses, the psychologist Dysart discovers that a reason why Alan Strange is having psychological problems is because he has grown up with his parents having conflicting viewpoints about religion. Alan's mother is a devout Christian who daily reads the Bible to Alan, but Alan's father is an atheist who antagonizes both his wife's religious practices and his son's religious beliefs. His father even destroyed the picture of the Crucifixion Alan had hung at the foot of his bed, which was apparently a very disturbing experience for Alan. Alan evidently had difficulty understanding how anyone could be as irreverent as his father. It was at this moment that Alan replaced his reverence for the Crucifixion with his reverence for horses.
Based on opening lines in the play, it's clear that Dysart understands how Alan could have developed a reverence for horses to replace his thwarted reverence for Christ. In his opening monologue, Dysart observes Nugget being affectionate with Alan and, yet, even more than affectionate. Dysart observes the horse nuzzling Alan's neck and can't stop thinking about what it is the horse "may be trying to do." He observes the horse trying to fulfill a desire that goes far beyond "filling its belly or propagating its own kind." He also observes that it's as if the horse is trying to kiss Alan but cannot due to the bit in its mouth or due, as Dysart phrases it, "its chained mouth." He observes that the horse trying to fulfill a desire through its "chains" is a type of suffering and wonders if it's possible for a horse to understand all of its suffering as grief, as we see in his lines:
Is it possible, at certain moments we cannot imagine, a horse can add its sufferings together--the non-stop jerks and jabs that are its daily life--and turn them into grief? What is grief to a horse?
Hence, clearly the author Shaffer is trying to portray horses as having human-like emotions, and understanding those emotions can have a powerful impact on humans. Since Alan too was able to understand horses as knowing suffering and understanding grief, he was able to liken horses to Christ, thereby enabling him to substitute his reverence for or worship of Christ with a reverence for or worship of horses, once his religious views became threatened by his father. Therefore, while the opening monologue tells us why horses can powerfully influence humans, the play itself tells us just how much humans can be influenced due to their emotions. Alan's father's persecution of Alan due to his religious beliefs in Christ imbalanced Alan's mind, leading him to develop an unhealthy substitution in which he replaced his spiritual beliefs in Christ for a reverence of horses that also led him to torture the horses, just as Christ had been tortured.
Horses have a powerful presence in Peter Shaffer’s Equus. We can even say that the horse metaphor is central to the novel. As we see, one of the major characters of the novel, Alan Strang, gives horses a "God like" status. Alan creates his own religion where Gods are nothing but horses that are controlled by Equus. When Alan has a sexual encounter in a stable, he feels deep guilt to have done a dirty act in the presence of a God and, thus, asks for forgiveness. He even blinds six horses by stabbing their eyes to escape Equus’ anger. Horses also have a strong influence in Alan’s sexual growth and maturation as well as development as a person throughout the story. Strong visual images of horses are depicted in the stories told by Alan’s mother. Historical and cultural associations of the four-legged creature having power are also shown. Hence, horses are equated with speed, strength, mythology, sexuality and power.