Your guess is as good as mine. The guide had said that most of the Pokémon found in Viridian Forest are Bug-types, but so far it's been mostly caterpillars and cocoons. Then there was just the one Pikachu.

[Athrun walks with his hands on his backpack's straps. He doesn't look at Yukari as he talks, instead focusing on keeping an eye on their surroundings.]