Swarms

Go to Pewter City then talk to this guy that's wearing orange and black at the PokeCenter. He will say that there is a swarm of _____ and a certain route. He also says that the swarm changes every 10 minutes. After that, a different swarm comes in on a different route.

Swarms
Below is an in-depth table on all currently known Pokémon swarms in alphabetical order.