There is no source for this old saying, adage, axiom, etc.
So, you can tell your husband that you did not make it up. Many people have used it.
"The outside of a horse is good for the inside of a man."
- Author Unknown -
http://www.quarter-horse.com/bye.html
The North American Riding for the Handicapped Association, Purina and several equestrian special need groups offer another “must”. Because the old adage, “The outside of a horse is good for the inside of a man” is so true.
http://www.cchexpo.com/entertainment.htm
"...the school founder had opined in 1889 that "something about the outside of a horse is good for the inside of a student"
http://books.guardian.co.uk/review/stor ... 89,00.html
"The outside of a horse is good for the inside of a girl."
http://www.extendinc.com/phoenix/
Green Acres Riding Center is a Christian organization and believes that "The Outside of a Horse is Good for the Inside of a Kid" (as quoted by Winston Churchill). Horses are also good for the inside of all of us!
http://www.greenacresridingcenter.com/
"The outside of a horse is good for the inside of a person"
~T. Roosevelt
http://www.televar.com/~gcaudell/title.html
President Ronald Reagan used to say: "The outside of a horse is good for the inside of a man."
http://www.charactercenter.com/myhorses.htm