According to a reddit post, those players who catch Eevee can evolve it into first-gen Jolteon, Vaporeon, and Flareon.
This trick is pretty simple and can be done by giving Eevee a different nickname for its respective evolution.
For Jolteon evolution give Eevee the nickname Sparky; for Vaporean give the nickname Rainer and for Flareon give it the nickname Pyro.
Pokemon fans with a sharp memory probably remember these names from the original anime where The Eevee Brothers were named Rainer, Pyro, Sparky as they owned a Vaporeon, Flareon and Jolteon respectively.
See this trick working (as a proof) in the video below uploaded by skinzfan602:
https://www.youtube.com/watch?v=SUQlID2qAdg