What is the origin of the idiom "have a dog in the fight" and when to use it?

The phrase "have a dog in the fight" comes from a violent and illegal activity called dogfighting, where two dogs are made to fight each other for entertainment. In dogfighting some people have personal interest in the outcome of the dogfight, either because they own one of the dogs or because they have made a bet on the outcome. Over time, the phrase "have a dog in the fight" has been used in different situations to mean that someone has a personal interest in the outcome of an event or decision.