What was Reb Brown wearing under the loin cloth?

I don't think either Yor or Ka-Laa were wearing anything under their loin cloths. Perhaps a thong as you say, but basically you see their bare asses wayyyy too much in this film.

As I said elsehwhere, if I were making a terrible caveman film, my opinion about how much bare ass there should be in the movie would be that there should be no bare ass in the movie.

But I'm not Antonio Margheriti, who had a vision for the film, a vision where we would constantly see Reb Brown's bare ass.

Help.

They call that the taint, but I did see something orange-ish that might have been leather or a thong.

You! Obey the fist!
RIP Heath Ledger

Hmm, it's hard to tell but trying to catch glimpses of what was under Reb's loincloth is what kept everybody's eyes glued to the screen throughout the whole movie.