There is no such creature.
Some conjurations have the indestructible trait and do not have an armor symbol; Objects with the indestructible trait cannot receive damage, and as a consequence do not have an armor symbol at all. If a creature existed that had the indestructible trait, it would likewise require no armor symbol, as it would not be eligible for damage.
Creatures and conjurations are the only objects that can receive damage, so no other type of card has need for an armor symbol.