I loved this song when it first came out. But don't hold that against me -- I was in the "I want a pony" phase that most American girls go through, and I wasn't quite six years old.
Through the lens of adult judgment, I see the song for the sappy mess it is. The girl loves her pony, named Wildfire. (Historical note: in 1975, many little girls who wanted ponies planned to name them "Wildfire.") One night, during a "killing frost," the pony "busts down his stall" and runs off. The girl runs after him, and they both die.
Note to idiot songwriter: killing frosts kill plants, not horses and girls. A nasty blizzard might have caused the girl and the pony to be lost forever, but a frost? Puh-leeze. And then at the end, the singer says he's going to join the girl soon. Result: SAP!
-- SAPster Randi