I don't know if this was just something that happened in the Toronto area or if the 'practice' was more wide spread but it seems that having a photo taken sitting on the back of a pony was somewhat traditional.
Below is photo of a very young 'me' 'riding' the pony and below that a photo of my father also 'riding' a pony. There is 20 - 25 years between the times that the photos were taken. Essentially, a man would walk the streets with his camera and a pony. Parents would have a photo taken of the children on the pony and pay the man for the photograph documenting the 'occasion.'
Do you have a 'pony' picture? Let me know.