Ddk’s closest, so I’ll give it to him to move us on
The answer is they’re all mythical Pokémon that you can obtain in-game but only in games AFTER theirs
Jirachi - hoenn, obtainable in BDSP
Keldeo- Unova, obtainable in SwSh dlc
Meloetta - Unova, obtainable in SV dlc
Everyone else - Alola, obtainable in legends Z-A
Noctowl
Eevee
Vespiquen
Emolga
Raichu
Gardevoir
Omanyte
Natu
Ninjask
Ampharos
Gallade
Igglybuff
Venomoth
Espeon
Yanma
Oddish
Uxie
Ursaluna
Pikachu
Shot in the dark. They all weigh less than Steelix?
Probably but that’s not it
You know the rules, and so do I 
@crossbat It’s your turn to give us a theme to guess!
Not visual.
Squirtle
Geodude
Magby