No, it's not really "Alien",
although it's not exactly normal, either.

The Story:

The Polydactyl cat is a feline being that greatly differs from most usual cats. I, for one, have a Polydactyl cat. Her name is Ducya. She's either a total sweetheart or a raving mad lunatic of satanic murder. She'll be cuddling with you one moment and trying to assassinate you the next. I also own a dog (breed: Border Collie), whose name is Teddy. Teddy is a kind hearted soul, but can be dangerous when angered. However, my Polydactyl cat, who is roughly 3/4 of a square foot (yes, small, I know,) can beat him up in seconds. She even scared away a pair of foxes from my backyard last year (2013).

Now, The Facts:

Polydactyl cats have a special "Feature", as I like to call it, but scientifically it's called a "congenital physical anomaly". This physical anomaly is known as polydactyly or polydactylism. Polydactyly is a combination of a greek prefix and suffix mixed into the English language: polys, meaning "many" or "more", and daktylos, meaning "finger". In Greek it would look like ?????????????. Anyways, it is a condition that dogs, cats, and even humans may contain, that grants them more toes, claws, or fingers than the usual amount per animal of their species. It also has an opposite condition called "oligodactyly", meaning fewer fingers or toes. Now let's focus this information on polydactyl cats: The average feline has five retractable claws per paw. The polydactyly condition never states exactly HOW MANY MORE additional claws or fingers the being will have. It just states that it will have more, so the exact amount can differ. My cat Ducya has six claws on each of her front paws and seven on each of her back. Noticing this, I did some extra research and found that in cats or dogs, the extra amount of fingers / claws per front paw are the same for both front paws, but different from the back. the back two are also the same (usually). In humans, however, it's usually just a one-handed thing (haha).

Other Effects of Polydactyly on cats:

I never found any proof online for this next fact, but I find in real life that polydactyl cats generally have shorter hair and for some reason lack the hormones or thingymabobers that make people allergic to cats. My dad has a severe cat allergy, yet while within a ten foot radius of most other cats he will immediately start sneezing, Ducya will only have the slightest effect on him if he rubs her all over his face. The same is true for my friend's polydactyl cat. Also, the two polydactyl cats that I know are smaller and feistier than most other cats.

Pets

Got a cat allergy but want a cat as a pet? Go find yourself a polydactyl cat. They're actually not all that rare. Just make sure that you have peace between the human and cat from the start, or things could get rough.