Minecraft Mob: Animated Armor
While they appear motionless to give off the false sense of security, they're all possessed by ghosts from inside the suits of armor. Being animated, moving suits of armor that appear in hallways on a display within Woodland Mansions high above, and Strongholds down below.
Always having a weapon on hand such as a sword, an axe, a flail, a spear, a mace, a hammer or a halberd in hand with a shield for the free hand.
They're slow when walking, and follow a patrolled path while keeping a watch for any intruders lurking within. Their armor does create sound when walking, as the typical clanging and banging of plating is reasonable.
Some, however, will come alive and ambush adventurers and miners without warning, sometimes. And being that they're enchanted suits of armor, they're ordered by their masters and given commands to defend their domains. They'll even guard doorways too, should any intruder get too close to a seemingly open way to the next chamber on the other side, where these suits of armor are present; since much of them are motionless, and some can't be trusted because as soon as they enter through the doorway, the sounds of their footsteps will be heard afterwards.
These hostile suits of armor serve as a cautious warning to other hostiles that they'll be ambushed, and will be forced to fight them off. They don't strictly target miners or adventurers alone: they target hostile mobs like Silverfish, for example, in the Strongholds too.
Armor piercing weapons like axes and maces will slay them, as they'll fall to pieces with no evidence of a physical presence having worn the suit of armor in the first place being present.
Strongholds are especially dangerous when these mobs are present, because no one knows what should be a mere display of armor, or a hidden foe that'll come out alive and attack.