A few of these Pokémon, as a result of new evolutions, become a part of evolutionary families with branched evolutions. Pokémon which evolve into a Pokémon of a previous generation, instead of from it, are always baby Pokémon.
Later generations have modified old families by introducing regional variants of older Pokémon adapted for life in different regions. Certain families are entirely comprised of variant forms while others share an unevolved state before diversifying, much like a branched evolution. Certain Pokémon will only evolve from a specific regional variant and, therefore, have no counterpart.
Generation IV introduced the most evolutionary relatives to Pokémon from previous generations, with 29, and Generation VI introduced the fewest evolutionary relatives to Pokémon from previous generations, with only one, Sylveon. The only generations that didn't introduce any new cross-generational evolutions (not counting regional variants) are Generation V and Generation VII.
Roselia is the only Pokémon to have both a pre-evolution and an evolution (Budew and Roserade, respectively) introduced in the same generation.
Porygon is the only Pokémon to have an evolution introduced in a later generation that was later able to evolve once again.