The Fire Plug (1937)

Scrappy is playing with the old gray mare who was the town's fire horse until being replaced when the town's fire department purchased a modern motorized fire truck. But when the fire alarm sounds, the old mare responds, and with Scrappy steering the discarded truck, they dash to the fire along with the fire chief in his new truck. After the new truck breaks down, the fire chief admits that the old gray mare is a s good as she used to be.

Unfortunately the movie The Fire Plug is not yet available on HBO Max.