Mutations cause alteration of pre-existing traits, and cannot create new structures de novo. E.g. winged horses. Types of mutations possible are constrained by existing developmental pathways. Most mutations are very nearly neutral. Some mutations that are disadvantageous in the ancestral habitat may be advantageous in a new habitat.
Many mutations are pleiotropic - they have more than one phenotypic effect.
Mutation as a random process All conceivable mutations are NOT equally likely. All loci, and regions within a locus, are NOT equally mutable. CAN affect mutation rates Mutation is probabilistic. The chance that a mutation will occur is not influenced by whether it would be adaptive to the organism - the environment does not induce adaptive mutations. It isn't new variation, but new combinations of existing alleles.