Tangrowth is the only resident Ditto meets in Pokémon Pokopia’s opening wasteland.
After Ditto awakens in the Withered Wasteland, it encounters a Tangrowth living alone there. This Pokémon later becomes known as Professor Tangrowth and serves as an important guide during the early story.
The setting establishes that humans and other Pokémon disappeared long before the game begins. Tangrowth therefore provides Ditto with information about the ruined world and helps frame the player’s mission to restore it.
Tangrowth’s identity is also a common mix-up because several grass-type Pokémon appear in the game’s habitat-building activities. However, the solitary resident at the start of the story is specifically Tangrowth, not Bulbasaur, Venusaur, or another plant-associated Pokémon.