The Albino Spider was cut from the final product of Thief: The Dark Project. It was first seen in the Thief 1 promo video and the skin textures can be found in the game files, labeled "spidera", likely meaning albino. These are modified, however, as the small yellow sewer spider uses the same textures. The other spider textures being labeled "spiderg" for green and "spiderR" for red.
Because the texture file shows a humanoid torso and a three-fingered arm, it may be assumed that the Spider Beast was previously meant to be very different from the one shown here.