I really enjoyed the horde video and how they constructed it in the game.
For those who haven't watched the video yet: the hordes are more an endgame thing that can still be tracked down early in the game, but they talk about how, through testing, people tried to take them on just because they're there, they're migrating, they're in the world, and they wanted to see how they could handle them. You'll probably just die early on, but it is pretty cool that, for a while, they'll be something to avoid or utilize to your advantage against other enemies.