RINO, means "Republican in name only." But where does this asumption come from that being a Republican is or ever has been some unmutable constant in the some 165 year history of the GOP?
As for me, whatever my opinion is worth, the GOP has been going to hell in a handbasket over the past 35 years. But when I was a far younger, there was much to admire about the GOP and it leaders. Intellecual leaders like Rockerfellar, Romney Senior, John Lindsey, Paul Tsongis, Dick Lugar, John Warner, Lincoln Chaffee, and gag even Eisenhower, were just a short list of GOP leaders who put America first and partisan politics a distant second. As that long dead GOP was at least respected as a minority by democrats as a minority party of principle. All those ex-GOP leaders I mentioned are all gone along with many I did not mention. As Dick Lugar as a lone hold out did not win his GOP primary race.
As there is a now a new definition of what a Republican is, and hence a new definition of a RINO. And by today's GOP standards, even Ronald Reagan would be defined as a RINO.