The black ibis mask is a part of the Black Ibis outfit introduced 23 August2011. Wearing this mask boosts thieving experience gain by + 0.8% (doesn't work in Pyramid Plunder).
This mask is received by looting urns while playing the minigame Pyramid Plunder. Wielding a Sceptre of the gods increases the chances of finding parts of the outfit.
The mask will not function as a desert disguse, despite the examination.
The snake on the top of the headgear is recognisable as being a Uraeus [The name given to the rearing cobra], which have been found on real-life Egyptian pharaoh's headgear. The most notable are the golden uraeus of Senusret ii, and on the mask of Tutankhamun.
Since the Black Ibis clothing is found in the Pharaoh's treasure rooms, as well as displaying traditional symbols worn by Pharaohs, it is logical to assume that the Pharaoh is part of (or even leader) of the mysterious Black Ibis organisation.