How does the author show that the Cyclops fits the definition of
monstrous?

The author show that the Cyclops fits the definition of monstrous by making a description of them.

Explanation:

At first we can see how the author wonders what the cyclopes will be like. If maybe they will be lawless savages or if they will be friendly.

But then he makes a description of them that would fit the definition of monsters. Let's look at the following quote:

"A grim loner, dead set in his own lawless ways.

Here was a piece of work, by god, a monster

built like no mortal who ever supped on bread,

no, like a shaggy peak, I'd say — a man-mountain

rearing head and shoulders over the world. "

In this way, the author manages to describe them as the beasts that he says they are.