The Esper Saga
Gargantuans are enormous insect-like creatures that dwell deep underneath the surface of Terra. Despite their savage and ugly appearance, the creatures are quite peaceful and highly intelligent – they live on great vine-like plants that stretch through great underground tunnels, feeding off mysterious yellow flowers that sprout from the vines.
The Ancients tamed the Gargantuans, and built great harnesses that attach to the scurrying creatures, allowing them to piggy-back ride the creatures between underground stations. The creatures are fortunately quite predictable in their movements – each seems to have only one or a few vines as its territory – so by luring one close, one can climb aboard the carriage and ride with it along the vine it calls home.