Alright, so I was on a pixelmon server last week and I was making a lot PCs in anticipation of the new breeding update (so I could craft as many ranch blocks as I could), and a porygon spawnned on the roof of my house. I was pretty sure that crafting the PCs caused it to spawn, so I investigated it more thoroughly. First I crafted stacks of PCs in a singleplayer world on the modpack, and then I tried it on only the mod itself. In both cases, I was able to spawn a porygon multiple times. I have also recorded and posted the spawning on youtube.
(the videos aren't top quality, since I'm having a little bit of trouble getting the recording software to work...)
Again, I hope that I'm not spoiling anything important and it was just added without indication to see how long it took before someone found it, and I'm sorry if I posted something I shouldn't.
Also, did I put this in the right section of the forum? I don't want to violate any rules with my first post on this forum (other than possibly spoiling a feature of the mod that wasn't supposed to come public yet).