The main metaphor in this story is, as the first answer says, thunder. The metaphor of thunder seems to be used just to describe the sheer physical awesomeness of the dinosaur, but it is much more than that. The thunder metaphor is also used to show us how any given action can have massive ramifications that are not seen. Any action, even one so seemingly innocuous as the killing of a butterfly, can “thunder” on and change the world.
Thunder can be thought of as a prominent repercussion of a previous event. The lightning flashes, but then the thunder comes and demands our attention much more than the lightning does. If you are turned the wrong way, you don’t see the lightning, but there is no hiding from the sound of the thunder. The thunder is only a result of the lightning, but in many ways it is more conspicuous than the lightning and has more of an impact on us.
The same is true in the story. Eckles’ actions do not seem very important at all. You could easily turn away from them and not notice. But what comes next—the thunder—cannot be ignored. Eckles’ action changes the whole of modern society. A small action has had tremendous repercussions.
Thunder is truly the major metaphor of this story, but it is not just about the way the dinosaur moves. It is also a metaphor for the impacts that our actions can have on the world.