Treecko first appeared in Pokémon Ruby and Sapphire and later Pokémon Emerald as a Starter Pokémon. In the Hoenn Pokédex, they are the first Pokémon listed. In every main game Geneeration III and up, Treecko could be obtained (usually through trading).
"A Wood Gecko Pokemon. It's a calm and cool creature that is not easily upset. It's also bold enough to stare down Pokemon larger than itself. It makes its home in large trees in forests, and its habit of fiercely defending its territory has some people calling it the "protector of the forest." It uses its thick tail to attack. When it levels up, it evolves into Grovyle."