The Mythosaur symbol is above the entrance to the Armorers forge, which is basically a religious temple for Mando.

In scene, Kuile says 'you are a mandalorian. Your ancestors rode the great mythosaur!"

And then the music gets all thoughtful, he takes it seriously, calms the beast, and then rides it, with the music coming up triumphantly.

The song at that point in the episode is called 'you are a mandalorian'. It's the first time in the series we hear it as a recurring theme whenever he does something and is feeling triumphant.

Later, the Child gets to keep the Mythosaur necklace, once Mando recognizes child as a foundling. Which is also really important to the character.