A monkey might grow two tails. Or none. Either would likely be a developmental problem, like a calf being born with two heads or an extra leg, or like those children born in the 1970s who were exposed to thalidomide in utero and born without arms or legs.
Not really related to evolution per se.
Yes, it is related to evolution. Evolution says that you can mutate into a new species without consequences, this is plainly wrong. If a monkey develops a tail it is not enough that the tail is simply there, the monkey must be able to use the tail.
Imagine I gave you a car and there was no-one to teach you what it was or how it worked. Do you suppose that you would be a successful driver on the basis of your own trial and error? I tell you no, you would have given up, be dead or in immanent danger of an accident because you are without any instincts or propensity needed to successfully drive the car.
What is more, this example of learning to drive a car is given in a context where you are
able to learn from experience, no such possibility exists in the womb or egg. If you make a mistake in the womb and you have no information to help you, no added DNA to direct you, you are as good as dead, whether now or soon after you are born. How then can evolution be thought to work so liberally - even as to create new species, for crying out loud - when so much can go wrong and probability dictates that it will? This is something that proponents of evolution cannot answer.
If I give you a piece of paper that says something on it in a language that you cannot understand, you are at a total loss as to what to do with it unless someone instructs you further. Suppose you are to deliver it to a certain address, where are you going to get the information for that address from if you cannot read the piece of paper you are given? I tell you that you will not even think to ask someone, if you do not understand that much, for why should you think it is even important?
Now imagine that it is not even part of your design to be able to read that language on that piece of paper and no-one before you has been able to either. Do you suppose that that piece of paper will be delivered? I tell you no, but it will go to waste. So it is with the expectation that something will come of mutations that is new and interesting.