Many texture pack makers leave their spiders, cave spiders, and endermen textures on default. This is because there is a glitch when you modify spider_eyes.png and enderman_eyes.png. In the image above, that is what a spider will look like. Strangely, the eyes still work.

Anyway, the way you fix this glitch is very simple, yet not so obvious. What you do is fill in the transparent parts on the spider_eyes.png and the endermen_eyes.png with black. What I mean is everything that isn't part of the image, change to black. One easy way is using the magic wand, but selecting manually also works. Remember, ONLY DO THIS ON THE EYE IMAGES! If you do this on any other image it will distort the mob. Hope this helps!

P.S. this texture pack will be released soon