At this point, we only have access to the western half of Route 2. Outside of a few patches of grass (with redundant Pokémon), there is nothing of interest on the actual route itself. But once you pass through the building, we encounter the first "dungeon" of the game: Viridian Forest.
Viridian Forest is big, long, filled with poisonous Pokémon and trainers who automatically battle you. If you aren't prepared, you could fall here. Make sure to pick up items you find laying on the ground and be sure to add to your collection. This is probably the closest part of the game to Tajiri's initial idea of Pokémon, searching in the woods for bugs.
Caterpie - Lv. 3-6 - Red/Green/Blue/Yellow 
While you can first catch them on Route 2 in Green/Blue, you'll be seeing these guys everywhere in Viridian Forest. You can also find their evolved form, Metapod, but it will only know Harden, making it difficult to battle with. It's better to evolve it from a Caterpie so it can continue to fight with Tackle. If you want a Butterfree, go right ahead and catch a Caterpie. Butterfree will be useful once it learns Confusion, as it can take care of Pewter Gym and Mt. Moon. But save for a battle against Erika later, Butterfree won't be cutting it as a long-term teammate. He's a must catch in Yellow though, as he learns all of his moves earlier and helps with the trickier sections.
Weedle - Lv. 3-5 - Red/Green/Blue 
Weedle is most common in Red, appearing on Route 2 in addition to the Forest, and it doesn't appear at all in Yellow (another reason why you should pick up a Caterpie there). Like with Metapod, Kakunas also appear in the Forest. Despite having the great Twineedle attack upon evolution, Beedrill can't really handle anything aside from Erika (maybe it can get a quick hit in on Sabrina, but it won't last long due to its part-Poison typing). Again, you can catch both bugs, or choose your favorite, but Caterpie probably would be the stronger decision of the two.
Pikachu - Lv. 3-5 - Red/Green/Blue 
While rare, Pikachu can be found in the Viridian Forest for non-Yellow players. While he doesn't learn as many level-up moves as his Yellow counterpart, he can do quite well with the library of TMs. Plus, this Pikachu can evolve into Raichu, making him much stronger in the long run! Ultimately, you'll probably keep Raichu on your team all the way to the end (helping you through Seafoam Islands and against Lorelei), which can't be said for the starter Pikachu. It provides some great balance to your team early on, so it's worth sticking around in the Forest in order to find one.
In addition to some new Pokémon, we also get to meet our first average trainers! They come in multiple varieties and each class has an identical sprite, making them similar to Nurse Joys, Officer Jennys, or Pokémon themselves! As a result, you'll have similar tactics for battling each type.
Bug Catcher 
This is our first encounter with a common trainer. Bug Catchers basically carry stronger versions of the wild bugs found in the Forest. In Red/Green/Blue, you'll find their collective teams consist of one Caterpie, one Kakuna, and four Weedles, with the final Lv. 9 Weedle being the strongest opponent in the Forest. Yellow Bug Catchers have a different arsenal, to reflect the lack of Weedles. They instead have three Metapods and five Caterpies, with a Lv. 10 Caterpie now being the final challenge. Although there is an additional Catcher added (possibly as a reference to the Samurai character from the anime) the challenge is lowered as Caterpies and Metapods go down a lot easier than Weedles (save for the additional Catcher's Metapod that actually knows Tackle).
Lass Yellow players get another additional trainer in the Forest. Lasses usually appear slightly later in the game and carry common Pokémon. This Lass has both a male and female Nidoran and for a while, I struggled to understand this addition. Unlike most of the changes in Yellow, this is not a reference to the anime. But I realized that since Nidorans are crucial for Yellow players to defeat Brock, it serves as a hint to new players who may have missed out on the early Nidoran to retrace their steps and catch one.
Once you're through the Forest, your next stop is Pewter City. We'll return to the hidden goodies on Route 2 once we have traveled across the map and received the necessary items.
Other Thoughts
- Replacing the Weedles, Kakunas, and Pikachu, Yellow players get more Pidgeys and a Lv. 9 Pidgeotto available for capture. This is to mimic the anime, where Ash caught his Pidgeotto in Viridian Forest (although there were also Weedle and Kakuna there too). As I said before with Pidgey, you're better off sticking with Spearow, although Pidgeotto is a stronger option than both, stat-wise. It will still take a while to be useful, but at least you don't have to wait for the first evolution.
- Although it doesn't effect this stage of the game, much later we'll see evidence in other canons of Viridian Forest stretching all the way over to Celadon territory. If you look at the map, you'll see that it's nothing but uninhabited forest between the two. I would have liked for a pathway to open up connecting the two later in the game, possibly revealing a new part of the forest that had stronger Pokémon (as seen in the various manga and anime). In Hey You, Pikachu, this area is revealed to be home to quite a few different types, including a Togepi! But I'm getting ahead of myself.
