Naga-ette bromide.
I am a item without use.
You do nothing with it, (SPOILER)just give it to the old man in 600 A.D.
I am guarded.
When you pick it up you are attacked by three henchs or something.
You cannot prove you got me without a screen shot.
It isn't shown in your inventory.
there are scales and skin on me.
I am not sure what bromide is (I'm not native english-speaker) but naga-ettes look scaly.