Game Compatibility

Currently, only Gen 6 and Gen 7 games have the ability for online access, thus it is only through these games can the Pokémon sold here be received at:

Pokemon X boxart Pokemon Y boxart Pokemon Omega Ruby boxart Pokemon Alpha Sapphire boxart
Pokemon Sun boxart Pokemon Moon boxart Pokemon Ultra Sun boxart Pokemon Ultra Moon boxart

 The Pokémon games have no backward compatibility. The basic logic is that a Pokémon originally caught, bred or received exclusively as an event Pokémon in a specific generation cannot be received at any past, non-same generation game version. Meaning, Pokémon originally coming from Gen 7 games (Sun, Moon, Ultra Sun, Ultra Moon) bearing the four-leaf clover or the + sign cannot be received at any Gen 6 (XY, ORAS) game versions.

It is also important to know that although they are of the same game generation, all Pokémon that were released only in Ultra Sun and Ultra Moon game (Stakataka, Blacephalon, Poipole, Naganadel, Totem Pokémon, shiny Ultra Beasts, Zeraora) can neither be received in Pokémon Sun nor Pokémon Moon.

Similarly, some specific Pokémon and Items can be received from Omega Ruby to Alpha Sapphire or vice versa, but not in XY. If an event Pokémon was released for only ORAS and was slated to be received only at these game versions, it cannot be received in other Gen 6 games such as XY, as X & Y are older game versions.

Conversely, any Pokémon released in older game versions can be received in any later version. For example, a Pokémon released or caught or bred in a Gen 6 game (X,Y, ORAS) with the pentagon sign can be received in any Gen 7 game. All Pokémon originally caught, bred or received as an event Pokemon from Generation I, I, III, IV, and V games can be received in Gen VI and VII.

It is therefore necessary to remember which game version is older, which ones are newer.

In a nutshell:

Game Version
 Release Date
Remarks
Pokemon X, Y October 12, 2013 receptive of Pokémon originally caught, bred, or event Pokémon received at Gen 6 (XY & ORAS), and past game versions; not receptive of Pokemon from virtual consoles. Items released only in ORAS (ex. Red Orb, Blue Orb) cannot be received in these games.
Pokemon Omega Ruby, Alpha Sapphire November 21, 2014
receptive of Pokémon originally caught, bred, or event Pokémon received at Gen 6 (XY & ORAS), and past game versions; not receptive of Pokemon from virtual consoles
Pokemon Red, Blue, Yellow Virtual Console February 27, 2016 not receptive of online trading
Pokemon Sun, Moon November 18, 2016
receptive of Pokémon originally caught or bred in SuMo and USUM 
(except Totem Pokémon, Stakataka, Blacephalon, Poipole, Naganadal, shiny Ultra Beasts, Zeraora), or event Pokémon received at SuMo and past game versions, including all virtual consoles
Pokemon Gold, Silver Virtual Console
September 22, 2017
not receptive of online trading
Pokemon Ultra Sun, Ultra Moon November 17, 2017
receptive of Pokémon originally caught, bred, or event Pokémon received at SuMo & USUM and past game versions, including all virtual consoles
Pokemon Crystal Virtual Console January 26, 2018
not receptive of online trading

POKÉMON GAME VERSIONS SERVICEABLE BY POKÉFELLA