No new information is gained by mutations, the information present is corrupted. I liken it to misspelling words on a page, one or two misplaced letters may not be a problem, but enough wrong ones and it is meaningless.
DNA doesn't contain meaning, however. It contains sequences, some of which are translated into amino acid sequences. Any mutation may change the resulting sequence of amino acids, and there's no limit to how much change.
That's where your analogy breaks down - all "words" in DNA are essentially equivalent. There are no "mispellings" because there are no three-letter triplet codes that do not either represent an amino acid or represent a start or stop codon.
There will always be new breeds of dogs, but they are fairly predictable as to what they will be: dogs.
There will always be new mammals, but there is no doubt that they will be mammals. There will always be new vertebrates, but there is no doubt that they will be vertebrates. There will always be new organisms, but there is no doubt that they will be organisms.
Evolution doesn't predict that dogs will not be dogs, any more than it predicts that a mammal will give birth to something not a mammal. Rather it predicts that the classification structures we invent to group organisms will become larger over time, and that we'll have to invent new subclasses. That's what an expanding, hierarcheal structure means.