You have a pup, congratulations. It's fast growth is going to amaze you.

Laurie is right, it's a pup. Just leave it alone and it will grow into another leaf. Many Sansevierias grow by spreading rhizomes under the soil. When they are ready, a new leaf breaks through the soil and it is called a pup until it grows into a new leaf.

My Sans generally do better when they are cramped, not to the point of becoming totally root bound, but pleasantly cramped. A Sans that grows via rhizomes can easily break a plastic growers pot, just split it on the side. So -- you don't HAVE TO repot right now. Also, winter isn't the best season to do so. But you may want to give your Sans a repot in the spring when growing season begins. Just make sure to use a pot with sufficient drainage holes (I always prefer more than one), a gritty and fast-draining kind of mix (I add perlite AND pumice, or pumice and chicken grit, to C/S soil), and consider using a plastic pot so the Sans can break through it in the future.

I have a plant that puts out pups this way every time. After a few months the pup will suddenly stand upright and start spreading its leaves.

I had one grow out of the bottom drain hole and grew about 5 inches before it started it's upward action. It wants to get as far away from it's competition for light as possible.