Inspired by recent discussion in the Scarlet & Violet thread and this comic from XKCD:
In short, humans are really good at deducing patterns in data and using them to make predictions. In most datasets, there will always be pattern-like trends that at first seem to be immutable rules, until some random variation breaks it, because it was never a rule to begin with. Just randomness.
And of course, Pokémon is no exception. Or rather, it's pretty much the poster child of this phenomenon. Almost since the beginning, fans have spotted "rules" Game Freak seemingly adheres to when making their games, and used them to make predictions for the upcoming games. Sometimes the pattern holds, sometimes it fails. This thread is dedicated to spotting them. We can count patterns that seemed solid until they were broken, and patterns that still hold true.
I don't think we can consider something a pattern until it has been followed thrice in a row without any deviation.
Let's also avoid negative patterns, as in "Until Gen ___ introduced X, X didn't exist".
So I'll start the list of broken rules from Gen IV onward:
Until Gen IV ...
These are the ones I was able to spot. Doubtlessly, there are many others. Have you found any, and do they still hold up?

In short, humans are really good at deducing patterns in data and using them to make predictions. In most datasets, there will always be pattern-like trends that at first seem to be immutable rules, until some random variation breaks it, because it was never a rule to begin with. Just randomness.
And of course, Pokémon is no exception. Or rather, it's pretty much the poster child of this phenomenon. Almost since the beginning, fans have spotted "rules" Game Freak seemingly adheres to when making their games, and used them to make predictions for the upcoming games. Sometimes the pattern holds, sometimes it fails. This thread is dedicated to spotting them. We can count patterns that seemed solid until they were broken, and patterns that still hold true.
I don't think we can consider something a pattern until it has been followed thrice in a row without any deviation.
Let's also avoid negative patterns, as in "Until Gen ___ introduced X, X didn't exist".
So I'll start the list of broken rules from Gen IV onward:
Until Gen IV ...
- Cross-generation evolution only adds one member to an evolutionary family.
- The legendary trio present in each generation are always of different types.
- The Grass-type starter is always the first Pokémon in the regional Pokédex.
- The final evolution of the Water starter is always bipedal.
- There will be a single "special version" of the base games released at a later date, combining (in theory) the best of both versions and a little more.
- Every generation adds a Water/Flying Pokémon.
- Every generation adds a Pokémon based on a cat (counting Espeon for Gen II).
- Every even-numbered generation adds a pair of new Eeveelutions.
- Each generation addds more Legendary Pokémon than the last.
- The regional bird is always Normal/Flying.
- Flying is always a secondary typing.
- New moves are never introduced mid-generation, as this would break compatibility between the new game and the base games.
- Abilities are set in stone, and don't change for any Pokémon between generations.
- After the Gen II stat rework, the base stats of each Pokémon are set in stone.
- A clearly discernible trio of legendary Pokémon (not box mascots) are always present.
- The base games of the generation will get one or more follow-up games set in the same region.
- Mythical Pokémon are only ever distributed in time-limited events or similar shenanigans involving spin-off Pokémon games.
- A defining feature of legendary Pokémon is that they never evolve.
- If a starter Pokémon has a dual typing, it will be retained for the entire evolution line.
- All Pokémon planned to be released throughout the generation are stored in the files of the base games, to ensure full compatibility.
- Every even-numbered generation adds a new Eeveelution.
- Every even-numbered generation adds cross-generation evolutions (debatable with PLA, though).
- Even-numbered generations add fewer Pokémon than the generation before them.
- Although it may evolve into a different typing, the regional bird always starts out as a Normal/Flying type.
- Every odd-numbered generation adds a new mushroom Pokémon.
- Cross-generation evolutions are only added in even-numbered generations (exceptions exist for pre-evolutions, however).
These are the ones I was able to spot. Doubtlessly, there are many others. Have you found any, and do they still hold up?