I’ve seen some real old movies in which the “fire engine” is a large boiler, on a wheeled cart, pulled by two or three horses. What was the boiler for? Did it power a pump that drew water out of the hydrants, or out of another fire-engine vehicle?
Just for you. 
It’s got some neat pictures of the kind of engines you’re talking about. They’re steam-powered water pumps. And fire hydrants were invented in the early 1800’s, so they apparently were used to pump from them, and not other tanks.
Romana
They could also have been used to draw water from a source like a pond or a creek, if available. As can modern fire equiptment, at least that owned by rural fire departments who might have to deal with a fire in a location not near municipal water service.
If yer gonna have fire horses ya gotta have a companion to guide the lead horse…
The ability to run with horse and carriage is the reason the Dalmatian was so widely used with the fire carriage of yesteryear. They would carefully and aptly guide the firemen through streets of busy traffic. In addition, it was observed that Dalmatians formed an amazingly tight bond with horses.