I live in Charleston, SC and pretty much everywhere is a water / grass type biome. I've fond that Muk and Grimer are friends with Hypno and Drowzee. If I see a Drowzee I know that that area can spawn Grimers. For instance, I've only caught three Grimers in the wild and I've only ever caught them at the two places I know where Drowzee is likely to show up at night.
Anyone else see a correlation between Drowzee sightings and Grimer spawns?