In Dallas there is another sports team with the name "Dallas Rangers" which in the 60's is what the team was called. The owners then called them Dallas Cowboys to avoid confusion. Originally they were slated to be known as the Rangers but a local minor league baseball team was already using the name Rangers and "there wasn't enough room in that town for the both of them" so they went with a common Texas dweller, cowboys.
Here is the history of the teams logo
Here is the history of the teams logo