Adult rust dragon

Iron Scent: The rust dragon can pinpoint, by scent, the location of ferrous metal within a mile of it.

Magic Resistance. The rust dragon has advantage on saving throws against spells and other magical effects.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Rust Metal: Any nonmagical or +2 weapon made of metal that hits the rust dragon corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to Damage Rolls. If its penalty drops to -5, the weapon is destroyed. Non magical Ammunition made of metal that hits the rust dragon is destroyed after dealing damage.

Multi attack. The adult rust dragon makes three attacks: one with its rusting bite and two with its claws. 

Rusting bite. Melee Weapon Attack: +11 to hit, reach 15 ft., 1 target. Hit: 19 (2d12 + 6) piercing damage The rust dragon corrodes a nonmagical ferrous metal object it can see within 5 feet of it. If the object isn't being worn or carried, the touch destroys a 1-foot cube of it. If the object is being worn or carried by a creature, the creature can make a DC 21 Dexterity saving throw to avoid the rust dragon's touch.
If the object touched is either metal armor or a metal Shield being worn or carried, its takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 or a Shield that drops to a +0 bonus is destroyed. If the object touched is a held metal weapon, it rusts as described in the Rust metal trait.

Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 13 (2d6 + 6) slashing damage. 

Breath Weapons (Recharges 5/6). The rust dragon uses one of the following breath weapons.

Acid breath. The rust dragon exhales acid in a 120ft line that is 10ft wide. Each creature in that line must make a DC 21 Dexterity saving throw, taking 90 (20d8) acid damage on a failed save, or half as much damage on a successful one.

Corrosive breath. The rust dragon exhales a reddish brown liquid in a 60ft line that is 5ft wide. Each creature in that line must make a DC 21 Dexterity saving throw or have all non magical metal items on them rust away to scarp. Magical items get a -1 penalty as if effected with the rusting bite or rust metal trait of the rust dragon.