The legendary Mandalorian beast seen in Season 3 is the Mythosaur.
In Mandalorian tradition, the Mythosaur is an enormous creature associated with the ancient founding of Mandalore. Legends say that the warrior Mandalore defeated and rode one, making the beast a powerful symbol of Mandalorian identity. Its skull became an emblem used in Mandalorian armor and across the Star Wars franchise.
Din Djarin first discovers that the creature may still exist beneath Mandalore's Living Waters in Season 3. Although the Mythosaur is briefly glimpsed underwater, it is not a rancor, which is another large Star Wars creature, nor a krayt dragon, a desert predator from Tatooine. Mynocks are much smaller, bat-like creatures that feed on power cables.
The Mythosaur's appearance also connects directly to the Darksaber and the question of who can unite the Mandalorian people. Its survival suggests that ancient Mandalorian history is more than mythology.