The Tortoise is a figment of Homer's imagination.
Homer meets the tortoise during his chili hallucination. He spells out "HELP" with rocks, and one of the rocks starts crawling away. Homer yells at it to get back in line with other rocks, and then sees it's a tortoise and it's very slowly spelling out a message in the sand that reads "Follow the tortoise." Homer does so, but quickly gets impatient with the tortoise's slow pace. He gives the tortoise a resounding kick, which sends it flying out of sight.
When Homer catches up to the tortoise, it's standing next to a small pyramid-like structure and pointing to it. Homer says, "You want me to climb that, huh? No problemo." But before Homer can even set foot on the pyramid, the ground rumbles and the pyramid very rapidly grows to several times its original size. Homer then says, "This is 'cause I kicked you, isn't it?" and the tortoise nods yes.
After a long, exhausting climb, Homer reaches the top of the pyramid and meets his Spirit Guide.