Nancy Drew and the Clue Crew, Pony Problems Book 3 by Carolyn Keene

This pony knows quite a few tricks! A new petting zoo has opened in River Heights. Nancy, George, and Bess can't wait to check it out - - the zoo has the "cutest" shetland pony named Buttons. And it looks like the Clue Crew won't have to wait very long to get a glimpse of Buttons - - he keeps escaping from his pen and showing up all over town! He's getting pretty famous for eating the flowers in everyone's front lawns. As much as Nancy secretly wishes Buttons would turn up on "her" lawn, she knows it's important to make sure he stays put. The only trouble is, no one knows how he's getting out. Sounds like it's time for the Clue Crew to saddle up and settle this pony problem!