Aslan

"Quick! Quick!" shouted Aravis. "We might as well not have come at all if we don't reach Anvard in time. Gallop, Bree, gallop. Remember you're a war-horse."

It was all Shasta could do to prevent himself from shouting out similar instructions; but he thought, "The poor chap's doing all he can already," and held his tongue. And certainly both Horses were doing, if not all they could, all they thought they could; which is not quite the same thing. Bree had caught up with Hwin and they thundered side by side over the turf. It didn't look as if Hwin could possibly keep it up much longer.

At that moment everyone's feelings were completely altered by a sound from behind. It was not the sound they had been expecting to hear—the noise of hoofs and jingling armour, mixed, perhaps, with Calormene battle-cries. Yet Shasta knew it at once. It was the same snarling roar he had heard that moonlit night when they first met Aravis and Hwin. Bree knew it too. His eyes gleamed red and his ears lay flat back on his skull. And Bree now discovered that he had not really been going as fast—not quite as fast—as he could. Shasta felt the change at once. Now they were really going all out. In a few seconds they were well ahead of Hwin.

"It's not fair," thought Shasta. "I did think we'd be safe from lions here!"

He looked over his shoulder. Everything was only too clear. A huge tawny creature, its body low to the ground, like a cat streaking across the lawn to a tree when a strange dog has got into the garden, was behind them. And it was nearer every second and half second.

The Horse and his Boy pg 72-73, Chapter 10 the Hermit of the Southern marsh.