Basque club that still adheres to a Basques-only policy, even if it has been watered down to the point where they'll bus in promising youngsters at an early age and claim they're Basque on residency grounds. Basically, the more hardcore footballing version of Yorkshire CCC. One overseas player* has played for the club, French Basque Bixente Lizarazu. They were one of the few clubs to eschew shirt sponsorship until financial pressures led to them taking €2m in 2008 for a three-year deal from an oil company. One of only three clubs never relegated from the Spanish top flight (see also Barcelona and Real Madrid).
Forced to change the name to Atlético under Franco as non-Spanish language was suppressed, but now only referred to as that by idiots.
* - Athletic changed what they meant by Basque in the early 2000s and so long as you lived in the area from a young age, that was deemed OK. Hence they bussed in decent talent to the San Mamés academy from all around and they now have Venezuelan centre-half Fernando Amorebieta on the books.