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 rare Pokemon in a certain route. he also says the swarm lasts for 10 minutes, after that, a new different swarm comes in at a different route.

Swarms change every 10 minutes.

Swarms
Beldum (Super rare)

Buneary

Carvanha

Chimchar

Croagunk

Jynx (rare)

Lapras (Uncommon)

Larvesta (Uncommon)

Lickitung (Rare)

Lotad

Mareep

Mawile

Mudkip

Murkrow

Nosepass

Oshawott

Piplup

Ralts

Sableye

Sandile

Scraggy

Scyther (Uncommon)

Shaymin (Super Rare)

Shinx

Slakoth

Sneasel

Snivy

Starly

Tepig

Torchic

Treecko

Wailmer