ROUTE 203 POKEMON
Starly - Levels 4-6 - Common
Shinx - Levels 4-5 - Common
Bidoof - Levels 4-6 - Uncommon
Abra - Levels 4-5 -Uncommon
Kricketot - Level 4 – Rare (Nocturnal)

ROUTE 204 POKEMON
Budew – Levels 4-5 - Common
Shinx – Levels 4-5 - Uncommon
Bidoof – Levels 4-6 – Common
Kricketot – Level 4 – Rare (Nocturnal)
Starly – Levels 4-6 – Common

ROUTE 204 CAVE POKEMON

Zubat – Levels 4-6 – Abundant
Geodude – Level 5 – Uncommon
Psyduck – Level 5 – Very rare

Above: Behold, a battle in Route 204. This route is blocked off at first because of the horrors you'll find within. Just look at that little green guy. He's so scary  

Route 204, to the north, is a dead end at the moment (you can’t progress past the cave at the end without Rock Smash), but it’s in your best interest to capture and build up a Budew or two. They’ll be extremely useful in the looming Gym battle. You can also try catching a Psyduck in the cave, but they are extremely rare. There’s an area where they’re common not too far ahead, so you’re probably better off waiting until then.

Route 203, to the east, leads to Oreburgh City. You’ll encounter your, uh, “friend” near the entrance to town. He’ll have one of the starter pokemon you didn’t choose, and he wants to challenge you to a fight with it! Fortunately, he’s still pretty inexperienced, so as long as you avoid having him exploit your weaknesses, you’ll be okay. There are several other fights for the picking here, too - they can be avoided by staying out of peoples’ line of sight. Before you enter the cave, swing around north and west to grab a Paralyze Heal from a pokeball on a small outlet. You might also want to try snagging Abra. Since Abras always use teleport as their first battle command, you’ll just have to throw a pokeball and hope luck is on your side. The little guy will be near useless until he hits level 16, so use the method mentioned above for Kricketot to have Abra earn EXP in combat.

CAVE TO OREBURGH
Zubat - Levels 5-7 - Common
Geodude - Levels 5-7 - Common

Inside the cave, you’ll run into an explorer who will bestow upon you HM08, which teaches Rock Smash. Unfortunately, you can’t do anything with it until you’ve got your first Gym Badge, so just hold tight for now. Keep heading right, fighting off misguided souls who may challenge you. You’ll see several Rock-type pokemon amongst their ranks, giving you a clue of what’s soon to come.  Eventually you’ll reach the exit to Oreburgh City.