Originally, cattle was imported from Spain when they decided to colonize Texas. During the Civil War the ranches were untended and the cattle drifted into the wild. They began to mate with wild cattle, and by the time everyone returned from the war, the South was overrun with cattle.
Many people took the opportunity and started claiming cattle for themselves. With millions of cattle in Texas though, they were practically worthless. But in the north, you could sell a cow for ten times the price. So people began to hire cowboys to take a cattle drive noth.
Cowboys would take huge herds of cattle up north to the railroads. From there they would be shipped to northern cities. This was a great business and resulted in an economic boom for the people of Texas.