Take another look at the sentence:
Tonya made the mistake of walking her boisterous bulldog Billy in high heels.
In high heels, a prepositional phrase, is describing Billy, the bulldog, when its target should be Tonya. When a modifier is next to a word it doesn't logically describe, it is misplaced.
To fix the problem, you should revise the sentence like this: Tonya made the mistake of walking her boisterous bulldog Billy while she was wearing high heels. Now we're sure which creature has on the inappropriate shoes!