He did not look up until Johnny pulled sharply on the reins and gave a startling whoop built around the word "Whoa." Sandy's bulging eyes got a full-front, close-up view of the "thing what set." He saw a wicked nose with a feeler about twice as high as he was.
He saw great, terrible, outspread wings and a long slim body.

It looked poised, ready to come at him and snatch him with one frightful swoop, as he had seen prairie hawks snatch little birds from the grass.
Sandy forgot that he was a tired, sweaty, head-hanging horse.

He forgot everything except the four unbroken legs under him.

He wheeled half away and went lunging up the far side of the little basin as if he felt the horrible creature close behind him.
Johnny's mind had been so absorbed by the airplane that it took him a few seconds to comprehend that Sandy was actually running away with him.

It took him a few seconds longer to realize that Sandy's jaw was set like iron, with the bit gripped tight in his teeth.