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 can feel more and more of your dick straining against the costume, then to your surprise your penis is sticking out of the costume. There must have been a fly in the costume you didn't notice because the costume's material is thick enough you surely couldn't have torn a hole in it. You lay in bed stroking yourself with one hand and rubbing your tail with the other. It feels remarkably real now, like the tail is real and you're a real merman now. Too bad you don't have a mermaid girlfriend here with you. Your tail starts flopping of its own volition as you bring yourself to climax.