Think is because in Texas where a lot of Spanish settlements and Longhorns are derive from Spanish cattle brought 500 years ago, not only Texas has Longhorns, Nebraska, Oklahoma, Wyoming, the Dakotas and Montana are other states with a high population of Longhorns.
True, are a lot of Longhorns in Texas...we breed and raise them too. But, they are found all over the USA and some other countries. They were more or less indigenous from early times to Texas when they were introduced from early settlers, etc. Guess Texas is a touch synonymous with "Longhorns"...lot of businesses in Texas have Longhorn head image for their business logo, including city of Ft. Worth. Are also lot of Angus and Brangus in Texas too.
