The Oxford English Dictionary defines "dark horse" as a candidate about whom little is known, but who unexpectedly wins or succeeds. That's not a bad definition. Of course, we can find a better one.
Todd Rose and Ogi Ogas, both accomplished in their field of psychology and neuroscience respectively, worked together to write the book Dark Horse. Long story short, they define dark horses as those bravely blazing their own trails through the wilderness.
Their central concept of dark horse mindset is that the pursuit of fulfilment leads to excellence. This is exactly opposite to the mainstream dictum that the pursuit of excellence leads to fulfilment. For generations, we are being pressured to follow well-defined and rigid ladder rungs. All types of institutions, medical schools included, curate their scheme of admission and compel students to customise their portfolios to suit their tastes. Students are doomed by the Hunger Games system of survival or competition. How can they not be?
As a matter of fact, there is a way. Go and read the book Dark Horse, follow your personal passion, and not the dictated path.