Challenge: Can mutations build new structures?

I’ll throw out another example (although I know it’s been a subject of controversy in the last decade or so) - the “Panda’s thumb”. Giant Pandas and Red Pandas are genetically more distant than one would expect based on their similar unusual lifestyles for members of class Ursidae. But both exhibit the “extra thumb” (a modified wrist bone) that is extremely useful for the gripping and grasping required by their diets.

A quick layman’s summary can be found here:
https://www.nature.com/news/how-the-panda-s-thumb-evolved-twice-1.21300

And here is a link to a recent PNAS article:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5293045/