I say if a Pokemon can also learn Dragon Pulse they may as well get Draco Meteor. That'll not only include the Dragon Megas but also a lot of Pokemon that are in the Dragon Egg Group. There's quite a few "Oddities", but I think most can be handwaved as such:
* Reptile: Squirtle family, Lapras, Feraligatr, Groudon, Heatran(?), Krookodile, Terapagos
* Serpent: Onix family, Serperior, Eelektross(?), Runerigus (or rather depicts one... apparently it counts)
* Dinosaur/Pterosaur: Aerodactyl, Tyranitar, Tropius, Cranidos family, Archen family,
* Mythic/Dragon Connection: Arcanine (normal & Hisuian)(based on Shisa which are somewhat dragonic), Lugia (based on Ryujin & possibly wyverns), Necrozma (original form is a dragon), Poipole (evo is a dragon), Iron Jugulis (based on a Dragon-type)
* Hulking Beasts(?): Nidoking, Nidoqueen, Rhyhorn family, Aggron
* Pulse Users: Lucario, Clauncher family, Armarouge; (Okay, so they have no reason to get Draco Meteor, they're more the exceptions that proves the rule)
* Mew: Mew