Mega-Fauna

Mega-Fauna are a species of titanic bipedal, randomly-generated monsters, which have a chance to appear on any Primeval world (Primeval Forest, Dark Primeval, Volcanic Primeval).

These monsters can destroy blocks. Field Generator is advised if you are building your base on Primeval planets that have Mega-Fauna.

Are they dangerous?
From their gigantic size and how threatening they look, you may assume that they are super strong. In practice, they are almost harmless to the player. These gentle giants spend their peaceful days digging the ground and accidentally knocking over the trees, and it is often safe for player to stand right near them. Mega-Fauna does, however, dig so well that it demolishes any buildings on its way. That will include the NPC towns and anything that isn't tile-protected.

It is unclear if this semi-peaceful behavior is intended or if the Starbound AI (designed for smaller monsters) is just so hopelessly braindead when managing a monster of unexpectedly large size.