What is key to realise in this story which is so much about manhood and the process of transformation that is necessary to become a man, Pepe actually begins by moving in the opposite direction. As he flees the responsibility of dealing with the murder he has committed, he reverts to an animal state, ending up crawling on the ground like a snake. Ironically, he thinks that committing murder makes him a man whereas the reality is that his flight results in a kind of human regression that is only halted when he finally faces his pursuers and his responsibilities.
This regression is charted by the loss of various artefacts that we normally associate with Pepe being a man. Note the way that first he loses his dagger, which is described as Pepe's "inheritance" as it belonged to his father. Then he loses his horse and is forced to walk, until finally, driven to distraction by thirst and pain, he forgets his rifle. Note how the loss of these objects is used to reinforce his regression in the following quote:
He drew his great puffed arm in front of him and looked at the angry wound. The black line ran up from his wrist to his armpit. Automatically he reached in his pocket for the big black knife, but it was not there. His eyes searched the ground. He picked up a sharp blade of stone and scraped at the wound, sawed at the proud flesh and then squeezed the green juice out in big drops. Instantly he threw back his head and whined like a dog. His whole right side shuddered at the pain, but the pain cleared his head.
The key simile to focus on is when he whines "like a dog." Using such animalistic figurative language cements the impact of the loss of his artefacts and how he is left being less than a man as a result.