Changelings are said to be the offspring of elves, fairies or trolls which are left in place of a human child they have taken. Blond, good looking, healthy children especially would be taken to fairyland, where usually they would be slaves.
People (according to legend) reported sudden changes in their children such as bad tempers and huge appetites as well as physical differences, sometimes the child became very pale, or very hairy with a change in skin tone. It is said that the substituted child would be enchanted to appear human, the only way to tell would be a difference in the eyes, such as a sudden change of colour. Quite scary one would imagine!
One explanation for the tales of changeling children with such 'deformities' is suggested by the high infant mortality rates of the past, it may have been easier or more accepted to believe in other-worldly explanations for children being ill, and dying. Cases have been reported over a vast period of time however, from before the 15th century up to the late 1800's - who can say for sure what had been going on?