The reader can assume that Twain is gullible and determined to succeed.
Because he bought the horse from the high-energy salesman, Twain was gullible. In the story, he is easily fooled by the salesman's tone. In the story, he says,
"I did not know what a Genuine Mexican Plug was, ... I would own a Genuine Mexican Plug or die." (392). Twain admits that he does not know what a Genuine Mexican Plug is, but he buys it anyways because he gets the impression that the decision will be the life or death of him. Once he bought it, he believes that buying will improve his life. However, as soon as he got on the horse, he was lifted into the air. After an old man told him the truth about the horse, Twain tried to get rid of the horse. However, nobody would buy it. He was so desperate that he "... tried to give him away. But it was a failure. Parties said earthquakes were handy enough on the Pacific coast- they did not wish to own one." (394). Not only that, but the horse cost him $27, the stall room $15, and the hay $250. Buying the horse turned out to be Twain's misfortune.
Mark Twain, at the time, was also ambitious. "... swept through the town like the wind! ... sailing puff of dust on the far desert... sat up gallantly and gracefully" (392). In the beginning of the story, Twain can be seen fantasizing what his future could be in store for him. At the end of the paragraph, "[he] was resolved to buy a horse." (392). The sentence implies that Twain wants to get started with riding through the desert as soon as possible. Leading the horse to the plaza also suggests that he is confident.