I've heard a quip from my wife, "When you hear hoofbeats behind you, don't expect to see a zebra." Essentially, you need to know that booth horses and zebras make hoofbeats, but zebras are rare (in the US). It'd be foolish to look for a zebra until you've ruled out the possibility of a horse.

In fact, it even has a wikipedia page: https://en.wikipedia.org/wiki/Zebra_(medicine)