People keep asking where we got 512 Pokemon from. This is the last time I'm going to explain it:
There are 493 Pokemon according to the Pokedex. 494 - Attack Deoxys 495 - Defense Deoxys 496 - Speed Deoxys 497 - Ground Wormadam 498 - Steel Wormadam 499 - Origin Giratina 500 - Sky Shaymin 501 - Fan Rotom 502 - Frost Rotom 503 - Heat Rotom 504 - Mow Rotom 505 - Wash Rotom
All of these extra Pokemon have their own slot in the game's internal code, so they were easy choices to include. The rest were:
506 - Hail Castform 507 - Sun Castform 508 - Rain Castform 509 - Ground Burmy 510 - Steel Burmy 511 - East Shellos 512 - East Gastrodon
These extra Pokemon forms are, other than Unown, the only ones that have distinguished forms/cards in the TCG. As for Unown, the 28 different letters/symbols of Unown aren't that different from each other, plus how many people care about the 28 different Unowns in the first place? Other than that, Overcast Cherrim is pretty much ignored by every aspect of the franchise, so it was expendable. And we certainly weren't including 18 different Arceus that look exactly the same, other than typing and some minor color changes, nor were we including Pokemon with even less important alternate "forms" like Spinda or the Pokemon with gender differences.