The birth of both these acronyms comes from the widely used marketing practice that makes some things easier to remember and stigmatize. The whole story was actually an invention by the modern, mainstream economic school.
On the one hand, the bad "students" of the PIIGS (Portugal, Italy, Ireland, Greece and Spain), conveniently placed in such an order to be "nailed" in everyone's mind as something repulsive, like a dirty pig. On the other, the "good examples" of the emerging economies BRICS (Brazil, Russia, India, China, South Africa), placed in such an order to remind someone that some countries are trying to build their economies wisely, brick by brick.
Of course, all this mainstream perception was created fifteen or twenty years ago, when the West was certain that neoliberalism would manage to conquer the whole planet. The big irony of the whole story, is that PIIGS were always playing by the rules of the neoliberal bubble-style economy, and, especially when the crisis hit Greece and eurozone, they became the scapegoat of this crisis.
And while the system managed to blame the Greek public for all the catastrophe in Greece, hiding the huge responsibilities of the bankers who were rescued at the expense of the taxpayers, no one really understands why Ireland should be stigmatized as being sometimes part of the PIIGS. The crisis there was clearly caused by the banks. How the Celtic tiger became suddenly a pig?