A creature that looks like it was made from a giant and a small used electronics store inventory
This large creature has metal plating fused onto its exterior, along with a third arm and a missile battery attached to its shoulder.
When first encountered this thing was caged and was only released by its captors when the fight turned against them. With its large size and metal armor it was a much tougher opponent then the party originally started fighting against.
Seemingly out of rage the thing shot its missiles at its feet, bringing it down and dealing a substantial blow to the party.