You run your hands up and down your tail, enjoying the sensation. It feels so real. You're so horny right now. You lay back in bed, rubbing your crotch with your hand. You could take the tail off, or at least pull it down to your thighs but you're enjoying yourself too much. You try to slide your hand between the costume and your stomach, but instead of sliding under the costume your hand slides right over it and over your crotch again, triggering a wave of pleasure through your body as you feel something that shouldn't be there.
Sitting up, you look just below the top of the tail where you see what you felt: a slit. You carefully probe the slit with your fingers and gasp with both surprise and pleasure. What the hell? Did the tail turn you into a mermaid? You realize you better do something. You can't exactly call 911 for something like this. Maybe your girlfriend will know what to do - it's her tail after all. You could try to call her. At the very least you should try to get the tail off. As you think about what to do, you continue to finger yourself. Maybe you should have some fun with this first...