Cutie marks have always been essential to ponies, both culturally and as a species. They represent a pony's magic, their uniqueness, their destiny in life; they're what make a pony a pony.  

 
A long, long time ago, the three pony breeds looked a lot different than they do today. They were far more distinct from each other: unicorns were long and lanky, the Earth ponies strong and hefty, and the pegasi small, feathered, and lean. Despite their differences, though, they were all connected by one thing: the cutie marks on their flanks.
 
Once called symbols, these marks were larger and more elaborate than their current-day counterparts, often stretching down a pony’s leg or extending up towards their back. Symbols bearing groups of objects were often in groups of five to seven, unlike their modern-day counterparts, which usually feature objects in groups of three. They were more ornate and abstract, leaving their meaning highly up to interpretation.
 
Although symbols are more strongly rooted in magic than in biology, they’re still affected by genetics and DNA. As ponykind became civilized, a simplistic cutie mark became more attractive than a complex one in many pony cultures, because it signified a clear, direct path in life. As time went on, ponies’ symbols grew smaller and simpler, and the term “cutie mark” gained popularity. The marks were simple, clean, and cute!
 
However, in some pony cultures, the changing of the times was resisted, especially in unicorn-dominated areas. Unicorns are notorious for their traditionalism and emphasis on purity of bloodlines, and many unicorns prefer complex, more “old-fashioned” symbols. The continued breeding of complex-symboled ponies eventually led to a gene mutation which is commonly referred to as the Twice as Fancy gene.
 
Twice as Fancy, or TAF ponies, have “cutie markings” throughout their body instead of just on their flank. True, pureblooded TAF ponies have cutie markings all over their body, but these ponies are incredibly rare, if not extinct, since they can only exist through repeated inbreeding. Modern TAF ponies have a standard cutie mark, but have additional markings, usually appearing on the face or neck. TAF ponies are generally uncommon, making up about 10% of the pony population, and the mutation is considered, rather ironically, desirable.  
TAF Cutie markings appear at the same time as a cutie mark, and, while they look pretty, don’t serve any additional function. Despite this, some ponies believe that TAF ponies have stronger natural talents.  
TAF ponies are most often unicorns, but the gene is present in all 3 breeds.
 
A notable example of a TAF pony is Sir Fancy Pants, named aptly. The descendant of a long line of traditional unicorns, he has strong TAF genes that give him his striking appearance. Both of his parents are TAF ponies, and so is his wife, Fleur de Lis. The Fancy family, hence their name, are very proud of their TAF genes, and they intend to pass them on for many generations to come. Fancy’s cutie markings include the “crown” marking around his neck and the “cape” marking on his back.
 
Tuff Love, the daughter of Fleur de Lis, inherited her TAF genes and has three significant cutie markings: her “black eye”, “nosebleed”, and scruff mark. She has no real pride in being a TAF pony, and finds that her cutie markings earn her more concerned looks than admiring ones, though she is curious as to where the trait came from. She’s an uncommon example of a non-unicorn TAF pony- pegasi actually have the least percentage of TAF blood.
 
Another example is Trixie Lulamoon, although her cutie markings are far more subtle. Her grandmother had TAF genes, and as a result, she has a star marking on her cheek.