The tail attaches to the back of your kart and, as you'd expect, can take out other racers with a quick swipe, as you can see in this screenshot.
Some of the screenshots also appear to show a flying Toad, but before some of you get carried away expecting a new power-up, look closely at the picture and you'll see strings attached to his arms. Shame.
Nintendo also confirmed you'll be able to download ghost data via SpotPass and exchange it with other players via StreetPass, plus the option to register StreetPassed players to your online list to race against them over WiFi Connection.
Mario Kart 7 is out in early December.