The Sock Snake
The sock snake, Natrix socus, is native to most of Europe. Its name stems from its distinctive, and rather bizarre, feeding mechanism. The sock snake, unlike any other known species of snakes, eats mainly acari. Specifically, those on the feet of humans. The snake will also swallow any skin parasites, but few of them get digested.
When feeding, the snake will dislocated its jaw, and try and gob up the whole foot. Then it will slowly suck out the acari, and other small invertebrates. This is a long process, and the nutritional benefits are small, but the snakes' metabolism is slow, and doesn't need much food. And of course, it helps that their hosts are consenting.
The sock snakes have always been considered beneficial in Europe. Over time, they have been integrated into cultural and economic trends, becoming one of the trademarks of the old continent, much praised for their beneficial effects. For some reason, the snakes refuse to reproduce as soon as they are exported out of Europe, and it has been estimated that up to half the tourism to Europe from outside is due in part to the sock snakes.
Biologists stipulate that it was the beneficial effects the snakes had on human feet that allowed them to quickly spread through the continent, and flourish. Why they cannot reproduce anywhere else, though, is still a mystery.