A pair of brother and sister that appeared in the UK around the 12th century, in which they turned up in a small village with completely green skin, speaking an indecipherable language and refusing to eat anything but bean pods
After a while they learned English and their skin lost its green tinge, claiming they’d followed a river of light, heard the sound of bells and then woke up outside the town of Woolpit
Theories claim the children to be of the fabled Hollow Earth, a parallel dimension or extraterrestrial abduction