Bore Worms.

Bore worms are small when they are just born but can grow to a maximum length of 3 kilometers once they leave their host. They live in tunnels that they make themselves. Their ability to bore holes through the earth is why they are called bore worms. The tunnels have become their new habitat where they give birth to smaller bore worms. 

The smaller bore worms have inherited the ability to survive alone and don't need to parasitize giants but they can't grow as large as the mother worm unless they kill giants and feast on their flesh. The parasite then turns into a predator.