Species with mutations that nature would have prevented.
Natural mutations, in general, are harmful and do not survive because they do not benefit the individual.
Geneticists love to experiment in their laboratories with rare mutations, using artificial selection, and seeing how the specimens are affected.
One example is what dog breeders do. They choose individuals with certain new features and reproduce them artificially selecting for those features.
Those dog breeds which are short-legged, like Basset Hounds or Dachschunds, have a mutation called achondroplasia, a common cause of dwarfism, which in nature would not have succeeded since carriers cannot even reproduce naturally by themselves. By having such short legs, the males cannot mount the females and need to be artificially inseminated.