Silvally is the fox/wolf like thing on the logo of sword and shield (Look at silvally without the mohawk on top)

The legendary for Sword is Metal Snake/Wyrm Creature

The legendary for Shield is a pegasus/mustang like creature

Silvally evolves from Type: Null who has ability Battle Armor.

Type Null has to be "merged" with a eligible pokemon to give it its "Armored" evolution.

Legendary are not on Sword and Shield Logo, it is Silvally because the "Silvally wolf" is the rendered symbol that appears on the armored evolution animation in the game. Picture a Mega Evolution animation but the Sword and Shield wolves instead of the Mega Evolution symbol instead. its similar to that but instead of a sphere around your pokemon when it evolves, armored evolution is like the armor "growing" on the pokemon.

Finally, Meltan and MelMetal are the third legendary in the game. Like Zygarde in Pokemon X and Y. Ties into lore of the game as the source of Galar's wealth.