Physical characteristics such as cloven hooves, horns, wings, feathers in place of hair, lion tails, and such are locked to their respective species. A unicorn cannot be born with wings, nor an earth pony born with feathery mane.
Color, however, has been spread from ancestral pegasus hybridization to every species of horse in the world. Bright hues pass down from parent to child as easily as long legs or curly mane. Behind the scenes, genes to conceive a foal that is a member of another species are carried through many ponies’ lineage; you can never tell when they will manifest.
The one constant is that a foal will be born as either an earth pony, unicorn, or pegasus. Something with multiple traits would be… a different creature entirely.
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