u/AdFit2543

The Mechonis, Xenoblade Chronicles. Before the story of the game began, the Mechonis and Bionis were locked in a fight to the death, until after one particular strike renders both of them immobile. Hundreds of years pass as civilisations soon develop on both giants, although an alliance isn't able to be achieved as the two races would start to fight, in which the culmination of the war results in the Mechonis being revived again to finish the century old fight once and for all.

Alexander, Final Fantasy 14. A giant Primal (god) that resides in one of the maps of the game, quite literally referred to as a Living Fortress, it is in an unexplained bubble with floating rocks surrounding it. Throughout the main story, it's never really explained, it's just a giant mech that is there, that is until you finish the story and unlock the raids of that Expansion and Alexander awakens due to an evil orginisation that seeks to use it's power to rule the world.

Fourth Chorus, Hollow Knight: Silksong. During your adventures, you pass by a giant mechanical looking being that's stuck in debris, but since it's in the BG and the place you're exploring is rather war torn, it can just be explained as 'enviromental story telling' as the Hollow Knight games have plenty of that. That is until you get a new power up and pass by the mech one last time that you notice the Pin that was lodged in it has vanished and it regains it's strength to push away the boulders, revealing just how massive it is.