Do you need a special cable to use Android Auto?
The answer, simply put, is "no."
Any normal data cable should work just fine with Android Auto. Occasionally you'll hear stories of issues with extra-long cords, but I've used any number of microUSB cables that have come with any number of phones, and they all worked fine.
You don't need a special USB cable for Android Auto, but do consider the length.
Currently, however, I'm using a 6-inch cable I got off Amazon and am just leaving the phone in the cup holder. This helps keep me from being tempted to mess with the phone while driving — definitely a good thing. (And contrary to what you might have read, it is possible to dump out of the Android Auto splash screen and use your phone like normal while connected to the car. And that's a really bad idea while driving.)
One caveat here: If you're using one of the new Pioneer head units, make sure the USB cable that runs from the back of the unit through your dash or wherever is connected to USB2. That's the input for Android Auto. USB1 is for Apple's CarPlay.
And that's that, folks.