Someone had written "Fred" in faint handwriting on the frame, identifying the child as my uncle Fred Schwartz, older brother of my Mom. Once I researched my uncle Fred's birth date, I was able to estimate when the photo was taken--a winter in the very early 1920s. Now I knew who and when, but not where or why.
Then I began asking my older cousins about the photo. One cousin explained that ponies were used as photo opps, something she remembered from her childhood:
Entrepreneurs would bring ponies around to residential neighborhoods in New York City and offer to photograph children in the saddle, for a small fee.Next, using street-view images on Google, I compared the brick background of the apartment building behind the pony with the brick on the building where the Schwartz family was listed in the 1920 Census. That building still stands, visible online. And it turns out my uncle Fred was photographed right outside his tenement on Fox Street in the South Bronx. Mystery solved, story recorded for future generations.
By the way, doing a search for images showing "children posed on ponies in New York City 1920" returns a handful of similar photos. And when I show this photo to New York-area audiences and ask about the pony, usually a couple of people remember seeing similar photos in their family's possession!