Final answer:
The automobile is not an invention of the cowboy frontier days; rather, it came later. The lasso, saddle, and revolver are all inventions associated with this period, utilized by cowboys who often adopted Mexican ranchers' tools and techniques.
Step-by-step explanation:
The invention that is NOT a product of the cowboy frontier days is the automobile. During the era when cowboys were prominent on the Great Plains, the core items used by cowboys included the lasso, saddle, and revolver. These items were integral to the lifestyle and work of cowboys. The lasso, derived from tools used by Mexican ranchers, was essential for catching livestock. The saddle was also a necessary piece of equipment for the long hours spent riding horses, and its design was influenced by earlier Mexican saddles. Revolvers became iconic for the image of cowboys, but they were not as prevalent as dime novels would suggest. Conversely, the automobile was an invention that came to prominence in the early 20th century, well after the peak of the cowboy era in the late 19th century.