Bradbury creates a sense of horror at this point in a number of different ways.
"The monster at first motion lunged forward with a terrible scream. It covered one hundred yards in four seconds." This part of the passage shows how terrifying the T-Rex is by using words like terrible scream, lunged and by describing its quick speed it can set an atmosphere of how terrifying the T-Rex really is if you were there.
"A windstorm from the beast's mouth engulfed them in the stench of slime and old blood." This sentence shows how big the creature is by using "A windstorm from the beast's mouth" to show that the hunters are so small that the breath of the T-Rex is like a very strong wind to the hunters.
"Eckles not looking back, walked blindly to the edge to the path, his gun limp in his arm, stepped off the path, and walked not knowing it, in the jungle." This sentence again shows the horror the T-Rex inflicts on the hunters, in this case Eckles, the passage shows Eckles so horror shocked that he doesn't even know what he's doing of where his feet are taking him. Bradbury also makes this part of the passage effective by using one sentence and breaking it up with commas, pauses at certain points to show how terrified Eckles is.
"His feet sank into green moss. His legs moved him, and he felt alone and remote from the events behind him." This part of the passage again is about Eckles but this time it portrays Eckles in a slightly different view, it portrays him as being different, remote, alone as if he were a blade of grass in a field of weeds.
"Trees exploded in clouds of leaf and branch. The monster twitched its jeweller's hands down to fondle at the men, to twist them in half, to crush them like berries, to cram them into its teeth and its screaming throat. Its boulder-stone eyes levelled with the men. They saw themselves mirrored. They fired at the metallic eyelids and the blazing black iris.