One thing I noticed with the Cake Twins was that Mister and Missus Cake were extremely nervous about why they, as earth ponies, created a pegasus and a unicorn. My theory is this: Mrs Cake is infertile.

Equestria has been shown to have similar, if not exactly equal, hospitals to that of Earth's; it would make sense that the fixes for infertility would still apply. There are procedures where a donated egg cell can be fertilized and implanted into a barren mother, and this scenario makes sense, considering the irregularities.

Pound Cake has pale fur, brown eyes, and brown hair alongside his pegasus wings; he has little in common with either of his parents. However, brown hair and eyes are dominant traits in real life, which could possibly apply in Equestria.

Pumpkin Cake takes after her father, with pale yellow fur and orange hair. However, her blue eyes are completely new, though the color blue, if similar to earth genetics, is a recessive trait.

Notice how neither foal takes after their mother; unless pink hair, pink eyes, and blue fur is recessive towards red hair, green eyes, and yellow fur, then such a thing is impossible under normal circumstances.

Perhaps the genetic mother of the Cake Twins was the child of a unicorn and a pegasus; that would more easily explain the Cake Twins' species.

Did I miss anything, or get anything wrong? If so, please tell me!