Jim Taylor is, hands down, the most underrated RB in NFL history. As Tough J notes, he was a close second to Jimmy Brown during his heyday, but he is never even mentioned in any discussion of the game's all-time great running backs. This is obviously because he's white, but also because mentioning how great he was might make a few drunk white fans question why there aren't any other running backs that look like him carrying the ball in the NFL now.
What is really sad to consider is that, if Taylor were coming out of college now, he would be lucky to be drafted. He certainly wouldn't be given a chance to carry the ball for any team, and his only chance to play would be to bulk up and become a blocking back, as so many other great white runners have had to do. As for Hornung, he would have even less of a chance in today's NFL, because he didn't have Taylor's size. He would have had to make it as a quarterback (his college position at Notre Dame), which in all liklihood he couldn't have done, and thus probably have been one of those white players who are cut multiple times and are lucky to spend time on the practice squad. There is no way in today's NFL that any coach would have the audacity to switch a white QB like Hornung to running back.