The Pokemon franchise has been around for three decades now and has one of the strongest fanbases of any in the gaming industry. With so many games across multiple generations, some rules are bound to spring up among the passionate fans about what is and isn't acceptable within a Pokemon playthrough.
Game Freak has obviously established a number of official rules within the Pokemon universe throughout the years, but not every rule is written in stone and mandatory for all to follow. Some are unwritten rules that aren't necessarily mandatory, but which are obligatory and expected among all the loyal fans, which new players will likely want to know to avoid facing judgment from others within the community.
Unwritten Pokemon Rule #1: No Hacked Pokemon
More recent technologies have made the idea of hacked Pokemon within the main series games a bit more challenging, but they were particularly rampant back in the Action Replay/GameShark days. A hacked Pokemon is any that was obtained via illegitimate means within the game. This can just be something rare or inaccessible, like hacking Pokemon Gold and Silver to get the GS Ball Celebi event, or it can mean a perfect EV/IV shiny Alakazam that is certain to make any 3DS meta battler cry.
Hacked Pokemon can be a fun thing to play around with, but the unwritten rule is to not actually bring them into competitive play in the Pokemon games. It can get a player banned, and it just generally bogs down the experience for everyone else.
Unwritten Pokemon Rule #2: Fair Trade
The ability to trade Pokemon between different games has been a staple of the series since the beginning. A large caveat of those trades, especially since the introduction of online trading, has been the honor system. The unwritten rule here is to never be dishonest when trading with another player, stranger or otherwise.
Sadly, many players have had experiences in the past with players offering to evo-trade, meaning to exchange Pokemon that only evolve through trade, only to never return to the evolved Pokemon or players who offer Pokemon with certain natures, EVs, or IVs, only to not deliver on that while accepting the other player's legitimate Pokemon.
Unwritten Pokemon Rule #3: Protect the Master Ball
The Master Ball is an iconic rare item within the Pokemon main series. In each title, the player can expect to get just one and can expect that Master Ball to catch whatever Pokemon they desire without fail. The unwritten rule is for the player to exercise frugality with this Master Ball, and not just fling it away at the first mildly difficult Pokemon to wander along.
The Master Ball doesn't necessarily need to be thrown at the mascot legendary in each game. Some other legendaries, like the roaming dogs in FireRed and LeafGreen, would be way better suited for a Master Ball throw considering their challenge level.
Unwritten Pokemon Rule #4: If It's Too Easy, Try a Nuzlocke
"New Pokemon games are too easy" is the phrase heard everywhere around all corners of the community. It's not necessarily an incorrect thing to say, but it is pretty trivial considering how well-known it is at this point that very few new Pokemon experiences are going to test the skills of even the more inexperienced gamers out there.
The solution for any player out there who loves Pokemon but wants something that actually requires a bit of thought is the Nuzlocke Challenge. A Nuzlocke is essentially a set of extra rules that any player can self-impose over their game to make things a bit more interesting. In a Nuzlocke Challenge, any Pokemon that faints in the party is gone and must be released, never to be seen again. That simple limitation can add an amazing amount of depth to easier main series titles like Pokemon Scarlet and Violet, and it's an unwritten rule to pass on this information to other players who want to up the difficulty in their Pokemon playthroughs.