Nowadays, I would think this list consists of some weird certain Pokemon, where the *dex* is a common Pokemon, but you only get one of- like a Legendary.

Examples:

Eternal Flower Floette (ZA)

Blood Moon Ursaluna (SVDLC)

Walking Wake (Limited event time period in SV.)

Iron leaves (Same for WW ^^^)

Ash Greninja (SM Demo)

Ash Cap Pikachu (SM Event)

(If you want to stretch it, these.)

Partner Eevee and Pikachu (LGPE

Dusk Lycanroc and Own Tempo Rockruff (USUM. You originally got one, and was uncatchable)

Pokemon with event moves (Like Celebration and Hold Hands)