When two airplanes fight it is called a dogfight, from where did this term come?

It derives from WW I, when the planes were small and flew fairly close to the ground and could be seen quite clearly. They had engines that made a growling noise as they were low revving compared with things like the later Merlins, and the manoeuvering looked and sounded like two dogs having a fight. Nothing much more to it.

Dog fight is a common term used to describe close-range aerial combat between two or more military aircraft. The term originated during World War I, and probably derives from the preferred fighter tactic of positioning one's aircraft behind the enemy aircraft. From this position, a pilot could fire his guns on the enemy without having to lead the target, and the enemy aircraft could not effectively fire back. When two fighter aircraft circled each other trying to achieve this position, it resembles two dogs chasing each other's tails.

https://en.wikipedia.org/wiki/Dogfight