Step 1.) Use your polygonal lasso to cut the tooth and paste it on a new layer. You can do this using ctrl+c to copy and ctrl+v to paste it.
Step 2.) Now drag your new layer to lengthen the tooth.
Step 3.) Use the Edit > Transform > Perspective to shape the cut-out piece so it is now looking like a real fang. Also you might want to use the eraser to help you shape the fang.
Step 4.) Merge both layer (ctrl+e), and use the Clone Stamp tool to cover the joints and uneven areas and give the tooth more form.
Step 5.) You can use the soft paintbrush (capacity around 20) to make sure it really blends in with other teeth.