It appears in the fossil record around 30 million years ago and may be the earliest member of the second great subfamily, the Borophaginae, the bone-crushers.
And that first subfamily, the hesperocyonines, had already died out about 13 million years earlier, unable to compete with both and bone-crushers and the arrival of the cats.
Now, tens of millions of years ago, some ancestral canid, whether it was Prohesperocyon or someone else, was the predecessor to the first of the three great subfamilies of canids.