While Seattle Seahawk fans enjoy referring to themselves as the “12th man,” that’s legally not the case.
According to Darren Heitner of Forbes, Texas A&M University own the trademark rights to the phrase.
Seahawks fans call themselves the "12th Man", but Texas A&M owns all registered trademarks for the phrase.
— Darren Heitner (@DarrenHeitner) September 16, 2013
The Seahawks and Texas A&M have had some legal battles over the phrase in the past.
Seattle even acknowledges on their official web site that A&M owns the trademark.