Who doesn't love Pokémon? An easy to pick up, impossible to put down game with bright, colourful characters and simple gameplay that hides a complex fighting system.
Of course, there's always people out there looking to complicate things. EV trainers, who use hidden stats to breed and grow the strongest Pokémon they can, are often ridiculed for the amount of time they put into something that initially makes little difference, but there's another type of fan who obsess to an even greater level.
If there's anything more expansive than the game world itself its the theories people come up with for it. Beginning small with rumours hard to substantiate in the early-internet age (Mew is behind that truck, the S.S. Anne will return) it's created a massive backstory to the accepted continuity.
The biggest theory is one that even turns the whole thing into a bit of an eco cautionary tale; noticing that in the Kanto region (where Red, Blue and Yellow, the games this article will focus on, take place) there's few characters that aren't very young, very old or criminals, people crafted the idea of some form of war, which tied into various throwaway lines throughout those games. In recent years this has expanded to be full on chemical warfare that led to the creation of the Poison type. And that could act as an in-world explanation for some crazy theories that link otherwise entirely distinct Pokémon, some of which from the first Generation of games we'll highlight in this list.
If it all sounds a little crazy that's because it is - this is rampant speculation that goes far beyond the remit of the game. But that doesn't make it any less awesome to hear how certain Pokémon could be related (in-world or out).