One of the most infamous conditions in old Equestrian medical lore is that of the draconequus, for as a species alone they are known as mysterious and dangerous creatures. The first part of the name, "dracon", means "dragon" and refers to how the mature draconequus will appear; four legs and two wings, often with a long body, not to mention its strong connection to magic. The second part, "equus", means "horse" and refers to its immature form that earned the species its place in pony medical journals.
There are many theories on the origins on the species. Some suggest that they are a breed of parasitic dragon, either growing its offspring inside a pony until the parasite takes over or swapping its own young for those of true ponies to let pony parents raise them. A lesser known theory is that draconequi are true ponies with a mutation that leads to a bizarre magical reaction in later life. Whatever the case, the "changeling" foals show only limited signs until the final stage begins and once this occurs it makes rapid progress, normally being completed well within 48 hours.
This condition is extremely rare, notoriously difficult to diagnose in its early stages, and in all known cases the metamorphasis stage has been permanent. The traditional way to deal with these creatures, due to the danger they represent, is termination.
[Author's Note: This was written before MLP:FIM's second season ended and the use of the word "changeling" is in its classic form of a fake child used to replace one kidnapped by faeries.]