Some creatures, the reptilian Lizardmen in particular, have tough, scaly skin which acts exactly like armour. This save can be variable. Lizardmen Skinks, for example, have a scaly skin save of 6+, while mighty Dragons could have a save of 3+ or more! The effectiveness of the scaly skin can be combined with armour, so a model with a 5+ scaly skin save and a shield would have a 4+ save. Note that scaly skin is an armour save for all intents and purposes, and may be modified by high Strength, etc.