Cats also have them. You sometimes see them in the corner of their eye, or partially covering their eyes when they’ve just woken up or are sick or something. They are sort of a translucent milky white.
It’s because they evolved in desert conditions, from what I’ve read. They can see well enough through them but it protects against sand and stuff.
Presumably yes.
But for a bigger picture question, are all of those others able to interbreed with each other, as well? If yes, they are basically like different races of human or breed of domestic animal. If not, it gets much more interesting in terms of classification.