If you use certain Pokemon on your team in the main storyline, you'll sometimes get a strange message alerting you the Pokemon is trying to evolve but cannot complete the evolution.

As many Pokemon in the Kanto Pokedex received evolutions in the second generation, it was decided that these evolutions would be blocked off until a certain requirement was fulfilled. This means no Kanto Pokemon can evolve beyond the number 151, whether by happiness or item trade, unless you have the National Pokedex.

To get the National Pokedex, beat the Elite Four and present Professor Oak a Pokedex with at least 60 Pokemon caught. Then, your Pokemon will be able to evolve whether it's into a Pokemon from a later generation or not.