Don't forget Marowak~
But, to clarify, we've done RBY mons that were OU in any gen, even if they were UU in RBY. So, Pokemon that only made OU in one gen, like Marowak, Magneton, Tentacruel, etc. have all be covered. So, we'd either be looking at Pokemon that existed in RBY but never hit OU in the four past gens, or Pokemon that originated from GSC and were OU at some point in GSC, ADV, or DPP. If that makes sense...