The heart of Gregg stopped, and he went cold.

It was not only the fear that his journey might be ruined, but the fear that something had happened to this magnificent creature beneath him.
He swung to the side in the saddle and watched her gallop.

Certain she went laboring, very much as though she were trying to run against a mighty pull on the reins.
He looked at her head.

It was thrown high, with pricking ears.

Perhaps she was frightened by some foolish thing near the road.