Less common, but possible: "Ponyboy" is a name given to a horse or pony. In equestrian contexts, an article could be written about:

If a girl owns a pony named Ponyboy, then "girl riding ponyboy" is a straightforward search for user-generated content (UGC) of that specific animal.

The most probable scenario is that this is a misquote of the famous novel The Outsiders by S.E. Hinton. The character is the protagonist. A common search or conversational phrase might be "girl thinking about Ponyboy" or "Sodapop riding with a girl."