>>59112976
Interestingly, there's a video where someone shows by modifying the game that pokemon traded from an NPC don't evolve when trading due to the way the NPC trades are coded (this seems to be a step back from earlier generations, since at least in Yellow there was an NPC that traded you a Machoke that did evolve into Machamp during the trade). Even if you remove the everstone the Haunter doesn't evolve. So the everstone was likely added after they realized the Haunter didn't evolve into Gengar like it should to provide a justification for it, because GF couldn't be assed to actually fix the issue with the code.