Mask of the Colossus is a mask you earn by finishing an activity rather than picking it up. Complete the Shrine in Shadow (a Wind Shrine) and it joins your gear.
Complete the Shrine in Shadow (a Wind Shrine). The Shrine in Shadow also awards the Armor of the Colossus dye for the Ghost Armor.
Adachi Dragon Mask
Adachi Dragon Mask is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Adachi Half Mask
Adachi Half Mask is a mask you get by finishing "The Sister Betrayed", from Masako's tales, part 9 of 9. It is a story reward, so playing the tale is the only way to add it.
Ancient Dragon
Ancient Dragon is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Avenging Lord
Avenging Lord is a mask waiting at a Gift Altar once you finish "The Broken Blacksmith" in Jin's Journey. Each main story milestone unlocks a different altar mask.