Riddles in the Dark # 2

Here's another curious riddle:

Thirty white horses on a red hill,
     First they champ,
     Then they stamp,
Then they stand still.

Don't worry about naming the reference from where this riddle was taken--if you know it, it's probably best NOT to say it... so people don't cheat and look up the answers instead of trying to guess them : )

Oh, the previous Riddles in the Dark question is still "unanswered"... in case you want to try your hand at that, too.