My first pet was a huge Collie that belonged to my parents before I was even born. They lived out on a farm, and having a good farm dog was important back then. His name was Bonzo, and I am not sure where they got that name from, but he was a beautiful dark tri-color rough coat Collie, and I loved him a lot.
As I got older and was able to play out in the back yard; Bonzo was my constant companion. I would put the closed end of the leash around his nose, and then snap the other end into it, and I now had a "bridle" for my "horse". Bonzo was fine with that, until I tried to get on his back and ride him; then he would sit down. I am sure my mom was chuckling to herself when I complained to her that Bonzo would not let me ride him, and I wanted to play "horse" with him.
She explained to me that dogs were not meant to be ridden, and told me not to try anymore, because it might hurt Bonzo's back. By then, I had pretty much got the idea that it wasn't going to work, anyway.
However, he did fine when I put my doll, Susanna, on his back; so that is how we played "horse" after that.