The green children of Woolpit.

"One harvest-tide, when harvesters were gathering in the corn, there crept out from these two pits a boy and a girl, green at every point of their body, and clad in garments of strange hue and unknown texture. These wandered distraught about the field, until the harvesters took them and brought them to the village, where many flocked together to see this marvel."

Wiki:

The Green children of Woolpit were two strange children who reportedly appeared in the village of Woolpit in Suffolk, United Kingdom, in the 12th century. Accounts are given in the chronicles of Ralph of Coggeshall and William of Newburgh.

The children were brother and sister. Though of normal appearance in other respects, their skin was coloured green, and they spoke a strange language. Initially they refused to eat, though they did eat pitch from bean pods and eventually got used to bread. Their skin also lost its green colour after some time.
When they learned English, they explained that they came from the 'Land of St Martin', which was dimly lit because the sun never rose far above the horizon. One day, while tending their father's herd, they heard the faraway sound of bells.They crossed a "river of light" and found themselves in Woolpit.
After some time the boy, who had always appeared sickly, died. The girl went to work in the local manor house, and later married a man from King's Lynn.

Theories

One modern theory has it that the mysterious Land of St Martin was merely the village of Fornham St Martin, approximately eight miles away (but further than many villagers would have travelled). The children's accent or dialect may have been sufficiently different as to be unrecognisable, but given the fact there is a common market at Bury St Edmunds, and any reasonable route from Fornham St. Martin to Woolpit is likely to have passed through Bury St. Edmunds, this should be noted, but seems unlikely.
The faraway sounds of bells may have been from the Abbey of Bury St Edmunds. The children may have got lost in the woods for several weeks, and as a result of surviving on berries their skin turned greenish due to iron deficiency; this would explain why the colour returned to normal when they adopted a normal diet.
Another explanation, put forward by Paul Harris in 1998, is that they were possibly Flemish children whose parents had been killed in a period of civil strife. Eastern England had experienced Flemish immigration during the 12th Century, but after Henry II became king, the immigrants were persecuted. In 1173 many were killed near Bury St Edmunds not far from the Fornham villages. He also suggests the children may have been from the village of Fornham St. Martin where a settlement of Flemish fullers who would have access to a wide variety of dyes existed at the time in question. The children may have fled from their village and ultimately wandered to Woolpit. Disorientated, bewildered and dressed in unfamiliar Flemish costumes, they would certainly have presented a very strange spectacle to the Woolpit villagers.
The colour of the Green Children could be explained by "green sickness", the name once given to anaemia caused by dietary deficiency. Once given a proper diet of food their colour returned to normal. Given the possible Flemish origin of the Children, a green dye to help camouflage them during a time when Flemings were particularly unpopular seems just as likely.