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Surely, Snorlax and Lapras have a mega evolution. I just can't see how they wouldn't.
I'd also say Pidgeot, something in the Oddish line, (Vileplume/Bellossom) Crobat, Dragonite, Salamence, Tentacruel, Machamp, Marowak, Slowbro, Slowking, Nidoking, Nidoqueen, Exeggcutor, Poliwrath or Politoed, Jynx, Gliscor, Wigglytuff...
At the very least, Snorlax and Lapras need to have mega evolutions, if they don't. Those are extremely popular Pokemon from Gens I and II. The others are popular Pokemon from earlier gens who they introduced into the game, and I feel like they probably have a hidden mega evolution somewhere too.