The Boeing 757-200 Combi was a rare type. Only one was produced for Royal Nepal. However, the engineering wasn't wasted as the front cargo door design was later reused for the 757PF for UPS.

The Douglas DC-9-20 was also uncommon. I believe only 10 or so were produced for Scandinavian. However, the engineering was reused. It's the wing of a -30 combined with a fuselage of a -10.

I have not been on extremely rare designs but a few less common ones. For example, the Boeing 747-200SUD on KLM, Boeing 737-200QC combi type plane flown by several airlines including Alaska Airlines, 747SP, DC-10-40 (42 produced), 767-400ER (37 produced), 757-300 (55 produced), A340-500 (34 produced), Fokker 70 (47 produced plus one prototype).