Mana/Natural Sheild

I've seen red skeletons let off a metallic sound and a blue flash, which means they probabaly either have one of the monster-one resistances. I'm not sure which one but I just now witnessed it while doing the Glasses Pouch RP for G1 in Rabbie Dungeon. Could someone look into this?

Oh yea Thats just Magic Deflector, Heavy standard, and Natural shield. Also note that if monsters or players uses defense with a shield it will ping and glow when being hit by magic or an arrow, similar to magic and natural.