I first encountered the phrase “wag the dog” in a movie with that title. Being like 10 years old at the time, I didn’t realize that it was an expression and never stopped to consider what it actually meant (this happens to me a lot). People argue about the term’s definition on the Internet, but I think the idea is that some lesser entity is uncharacteristically controlling a larger one. So if an intern is manipulating his/her boss, that would be an instance of a tail wagging the dog.
Now to a Spanish word that implies a tail-wagging entity. Colear is a verb that means to wag its tail, and it comes from the noun cola, which can translate to tail, line/queue, and a few more vulgar things. The verb is used more excitingly in the figurative sense: past events can wag their tails, i.e. have lasting repercussions. So, for example, you could say something like “the Iran-Contra affair still wags its tail.” El escándalo Irán-Contra todavía colea. Amazing, no?
For those keeping score at home: cola (tail) comes from the Latin cauda (tail), which is also where we get the English-by-way-of-Italian coda (a concluding section of a musical work).
Here’s a floating dog with a spinning tail: