They are , according to biological nomenclature and classification, Bison...but no one calls them that..they are called in common parlance, and have always been called in common parlance, buffalo....cf Buffalo Bill, Buffalo Soldiers, Buffalo, New York, Buffalo , Texas, Buffalo Wallow, etc etc......
Insisting that they are bison is like insisting that possums are actually o'possums, or that crawfish are actually cray fish..nobody but yankees or pedenatic assholes would give a rat's ass anyway...