
Extra nipples usually show up in what is called the "milk line," or a line of embryonic tissue corresponding to where the nipples show up on your pet mammal. They're not usually quite as fancy and involved as the ones used in advertising. They might look like moles.
Men's nipples are generally considered to be useless, but in fact a man can lactate, in the right mood. That mood being if he'd been put on Thorazine, or had a pituitary tumor, or had a baby suckling on him night and day for a few weeks, or was a Dayak fruit bat. These are all pretty specific moods.
People wonder why men even bother having nipples, unless they're Dayak fruit bats where that kind of thing is just expected; and the answer is that early on, both male and female fetuses are pretty much alike, and the male fetus is just holding onto the possibility of being female, until it has to give up its aspirations and live out a life of anger and resentment.
(Similarly, females still retain a little bit of the vas deferens sperm chute too, although they have been socially conditioned not to ejaculate, especially in the boardroom.)

I believe someone grew the ear on the mouse on purpose to give it to someone else later. There is a man who had a second nose grafted onto his forehead to replace his own damaged one. There are people with two heads. And many people have been born with extra limbs. If I had an extra arm I'd want it in the back and use it just for wiping. But in a society that gets worked up about visible panty lines, such an innovation is not likely to take off.

I'm not wild about my eyebrow mole, but a real, committed horn would be cool. Not those sissy unicorn horns, either. You stick a drill bit on a horse and people just go all to pieces. And all the glitter that comes with it is a nuisance. No. I want a bugle right on my forehead. You show up unprepared for an argument with a bugle on your forehead, you're still going to win.