'Dragon earth worm
This is an ancient monster's husk, when they perish, they solidify their skin using all of the earth mana within their body to protect the eggs within their body. While this may seem dangerous, it takes a hundred years for their eggs to be able to hatch if the conditions are correct.
Their powerful heart mana manipulation allowed them to travel and condense their tunnels under the ground. This allowed them safe passage and some monsters often used these passages for many years afterwards due to the strength and safety from other monsters within them.
These dragon earth worms are often very small until they can get a single piece of dragon scale or dragon material in general. Afterwards, their bloodlines will awaken and they will grow rapidly over the years to become extremely larger. From there they will specifically chase and hunt other monsters with earth dragon bloodlines.