How is the emperor of Lilliput nearly injured as he approaches Gulliver?

The emperor's horse rears up at the sight of Gulliver.... as a result, the emperor might been thrown, but he wasn't.

The emperor was already descended from the tower, and advancing on horseback towards me, which had like to have cost him dear; for the beast, though very well trained, yet wholly unused to such a sight, which appeared as if a mountain moved before him, reared up on its hinder feet: but that prince, who is an excellent horseman, kept his seat, till his attendants ran in, and held the bridle, while his majesty had time to dismount.