Every league has the same top four or five teams stacked at the business end of most brackets. That's exactly why picking them rarely wins anything. The players who finish top tend to have one or two outsiders carrying real weight in their picks.
What makes a good dark horse
It's not just "a team that might surprise." It's a side with a kind draw, a settled core and a path that doesn't run through three contenders. Get those three things lining up and an outsider in your semi-final is a calculated edge, not a wild swing.
Pick one. Commit to it. A dark horse you half-believe in is worse than useless – it's the ones you ride all the way that separate you from the pack.