If it's name the dead-as-in-name-gone (U.S.) airline, the mainline ones I've flown on are
PS (acq'd by US)
PI (as above)
HP (body snatched US)
US (USAir + later USAirways incarnation - body snatched AA) - don't think I even flew Metrojet (ticketed but scuttled by 9-11)
QQ (Reno - acq' by AA)
PA + PA Shuttle
TB (became US shuttle)
CO (bodysnatched UA)
WA (acq'd by DL)
NW (same as above)
AQ