In the book Johnny Tremain, what metaphor does the author use to describe Percy's brigade?

After the battle had begun between the rebels and British forces, Percy's brigade marches to Lexington and Concord from Boston. The author writes, "the heavy dragon marched on its thousands of feet," and, later, she describes the brigade as "this great scarlet dragon" (page 224). She uses the metaphor of a dragon to describe Percy's brigade. Johnny Tremain notes the force and perfection of the brigade as they pass--the way in which every button is sewed onto their uniforms properly and every buckle is in the right place. Every box of cartridges holds just the right number of cartridges, and every musket also has a bayonet. Every horse has four shoes. The army, clad in scarlet, looks so powerful and perfect that Johnny is afraid about what they will do to the "untrained, half-armed farmers" (page 224) who are defending Lexington and Concord.