Ok I got a new idea.
There's no older pokemon evos in this generation. ok
Isshu is far away from every other region. So what if, some older pokemon that somehow were living in Isshu, mutated, adapted to current region. That's how Tauros became Baffuron and Luvdisc became Mamanbou. I still dont know how would it work in the games, but it's an idea which I think could explain the similarities between the pokemon.
ok i can live with that, or could be something like parallel evolution, like australia marsupials.