>>45831666Fall into the same trend as Pseudos/Legends in that they're highly desirable for other factors, in those cases because of how you get them.
Porygon could only be gotten at the Game Corner and was the highest priced reward, so naturally people are going to look at that and think "It must be really amazing if it's that hard to get!". It was essentially a mythical in its own right and people who grinded up all the way to getting it were incredibly prideful.
Milotic was essentially a playground rumour come to life. It's on the same level as shit like "MEW UNDER THE TRUCK". Imagine some kid telling you "yeah, I got one, you can only get feebas in this one route but only on four specific random tiles and then once you catch one you gotta feed it pokeblocks until its beauty is maxed out before levelling it up to evolve it", it sounds hella made up. And that's not even getting into the fact that without easy internet access most people didn't even know about the four tiles thing, so it was a huge mystery why some people were able to find a Feebas while others seemingly couldn't no matter how many times they tried. Since the other Anon mentioned Toxtricity, a big part in its popularity was also played up by the mystery regarding which form you would get when it evolved. Solving that was arguably one of the biggest parts of this Gen's leak/hype season and it brought a lot of attention towards the designs which only served to get people more interested and attached. People love interacting with others to figure out puzzles and Pokemon practically owes its success to that very concept - all the weird little tricks and myths people shared with each other to get all the Pokemon. Even Pikachu owes its success to the playground rumour of kids finding this rare Electric type in Viridian Forest.