The Mythosaurs were giant reptilian creatures that inhabited Mandalore. They live up to their name of being Myths as we do not know what they looked like outside of skeletons.

The Taung (original Mandalorians) came to Mandalore and made it their new home. They soon found out that the planet's apex predator was the great Mythosaur. Witnessing the power of these creatures, the Taung used the hunting of this creature as a right of passage.

Eventually the Mythosaurs were driven to extinction. However, their bones were made into weapons and armor such as the Mythosaur Axe and the first Mand'alor helmet. Their skulls quickly became prominent symbols which lasted for millenia among the Taung and Mandalorians.

Their size was impressive but was often exaggerated. Two Mandalorians attempted to create a theme park that featured an overly sized Mythosaur skull. The Imperial occupiers at the time thought it was a spectacular structure though.

Their design has been theorized and created by many fans. Some think that they were dragon like herbivores while others think they were T-Rex like carnivores.
(Fan art by LDN-RDNT on ZBrush)

The Mythosaurs were recently brought up in the first season of The Mandalorian. It was said that the Mandalorians rode the Mythosaurs before their extinction. It is unknown if they were hunted by the Mandalorians or if they were just used as mounts.