In order to get to Route 103, need to take Route 101 to Oldale Town. With ledges blocking the way, you must go through the tall grass to keep moving north. Pokémon can be battled, but not captured, seeing as you don't have any Poké Balls yet. It is possible to train if you wish to raise your level to at least 7 or 8 to make it easier to face May/Brendan.
On this route you will find Zigzagoon, Poochyena, and Wurmple. For Ruby and Sapphire players, Poochyena will be a little more difficult to find than the other two Pokémon, and for Emerald players you may have more difficulty finding Zigzagoon. Head north into Oldale Town.
New Pokémon
#263 Zigzagoon
This Pokémon is pretty common throughout the land of Hoenn, so finding one shouldn't be a problem (except Emerald trainers, who have hordes of Poochyena to deal with). Once you get Poké Balls you should try to snag this raccoon-like critter, since its Pickup ability is one of the most useful abilities in the game. Pickup gives you a 10% chance to gain free items after a battle, even if you don't use Zigzagoon in the battle. Emerald players get the added bonus of a more developed Pickup system, allowing them to gain better items the higher its level. Zigzagoon evolves into Linoone at level 20, which also has Pickup.
#261 Poochyena
#265 Wurmple
Poochyena aren't as common in Ruby and Sapphire as they are in Emerald. Their specialty is being a Dark type, a relatively rare variety of Pokémon, which gives them immunity against Psychic Moves. It learns Howl at Level 5, which ups your attack when you use it, allowing you to defeat opponents faster. Unfortunately, Dark type moves do not get powered up by Howl. It evolves into Mightyena at Level 18. Mightyena, however, is weaker than most of the fully evolved Pokémon, so using it later in the game isn't recommended.
This little guy is the main variety of bug in the Hoenn region. At first they're nothing too special, but when they reach Level 7, something special happens. This bug can evolve into either a Silcoon or a Cascoon. You'll probably want to catch its other form rather than just another Wurmple, since what they evolve into is random.
Poochyena
Zigzagoon
Wurmple