In those days, they were called 'Gonin bugyo,' but it became quite confusing with 'Gonin gobugyo' during the Edo period (though they were mainly called 'Gonin otoshiyori' at the time); as a result, the term 'Gotairo' was coined later to indicate the former to distinguish between them.
なお、当時は「五大老」の呼び名は無く、「五人御奉行」などと呼ばれていたが、江戸時代にいわゆる五奉行（こちらは主に「五人御年寄」などと呼ばれていた）と混乱し、後に「五大老」と呼ばれるようになったものである。