Torterra is based off the Cosmic Turtle, or "World Tortoise". The Cosmic Turtle is a large mythological turtle in Native American and Chinese legend that swims through space carrying the world and everything living in it on its back. 

Torterra fits well into this because the Pokemon has the ability to carry a little ecosystem on its back. Some smaller Pokemon spend their whole lives on its back without Torterra even realizing, or even caring.