Original term (referring to no formal organization) was BRIC, coined in 2001. BRICs common as a plural form, as in "the BRICs" (which I recall seeing), but not BRICS. The four countries started holding summits in 2009, and in 2010 declared South Africa part of the group, at which point a specific sense of the term (referring to invitees to these summits and members of related efforts or organizations) became BRICS.
https://en.wikipedia.org/wiki/BRIC
Original term (referring to no formal organization) was BRIC, coined in 2001. BRICs common as a plural form, as in "the BRICs" (which I recall seeing), but not BRICS. The four countries started holding summits in 2009, and in 2010 declared South Africa part of the group, at which point a specific sense of the term (referring to invitees to these summits and members of related efforts or organizations) became BRICS.