I know it's not what you mean by "pokemon that are related", it makes me think of
A) version exclusive or limited pairs, like Ekans and Sandshrew, I forget for example right now what Meowth pairs with in rby but right now I re-remembered it's Mankey. Or Oranguru and Passimian is one.
B) Pokedex entries with other pokemon. Raichu knocking over a Copperajah always makes me go "oh yeah that's a thing, they have a relation" or Onix disturbing Diglett tunnels and such
GTM - Boko United
survivor and dillos and nintendo and you are awesome