Animal-assisted therapy sprouted from the idea and initial belief in the supernatural powers of animals and animal spirits. First appearing in the groupings of early hunter gatherer societies. In modern times Animals are seen as “agents of socialization” and as providers of “social support and relaxation.”  Though animal assisted therapy is believed to have began in these early human periods it is undocumented and based on speculation. The earliest reported use of AAT for the mentally ill took place in the late 18th century at the York Retreat in England, led by William Tuke. Patients at this facility were allowed to wander the grounds which contained a population of small domestic animals. These were believed to be effective tools for socialization. In 1860, the Bethlem Hospital in England followed the same trend and added animals to the ward, greatly influencing the morale of the patients living there. Sigmund Freud kept many dogs and often had his chow Jofi present during his pioneering sessions of psychoanalysis.
He noticed that the presence of the dog was helpful because the patient would find that their speech would not shock or disturb the dog and this reassured them and so encouraged them to relax and confide. This was most effective when the patient was a child or adolescent. The theory behind AAT is what is known as Attachment theory. Therapy involving animals was first used in therapy by Dr. Levinson who accidentally discovered the use of pet therapy with children when he left his dog alone with a difficult child, and upon returning, found the child talking to the dog. However, in other pieces of literature it states that it was founded as early as 1792 at the Quaker Society of Friends York Retreat in England. Velde, Cipriani & Fisher also state “Florence Nightingale appreciated the benefits of pets in the treatment of individuals with illness.
The US military promoted the use of dogs as a therapeutic intervention with psychiatric patients in 1919 at St Elizabeth’s Hospital in Washington, DC. Increased recognition of the value of human–pet bonding was noted by Dr. Boris Levinson in 1961”.
Wikipedia – Animal-assisted therapy