Tail

The tail wagging the dog
A situation where there is a reversal of roles, with a small or minor element of sth having a controlling influence on the most important element.
"If you let your children decide on everything, it will be a case of the tail wagging the dog."

On somebody's tail
If you are on somebody's tail, you follow them closely.
"The suspecct hasn't been arrested yet, but the police are on his tail."