A monic polynomial has a leading coefficient of 1, not -1.
You may wonder why this is the case, but it is defined that way to make some of the theorems simpler.
For example, when you define the GCD (greatest common divisor) of 2 polynomials, you want to get a unique answer, so
the GCD of two polys (as opposed to just
a GCD of two polys) is defined as the monic polynomial of greatest degree that divides both the polynomials evenly.
http://en.wikipedia.org/wiki/Greatest_common_divisor_of_two_polynomials
If polynomials with a negative leading coefficient were allowed to be monic, then you still wouldn't have a unique GCD.