A tiny yolkless egg shows up in your henhouse. Today, we know this to be a chicken’s first training egg. In the 12th century? It came from a rooster, and you better throw it over your house or it will be born a monster.
This egg is known by a number of enchanting names: wind egg, dwarf egg, cock’s egg, and witch egg. It’s basically the egg equivalent of training wheels – a young chicken will lay a small egg without any yolk as a way of getting its egg-producing parts in order.
In Medieval Europe, however, it was a bad sign. Folklore had it that such an egg was laid by a rooster. I’m not sure where the egg would have come out of said rooster, and folklore is suspiciously silent on that topic.
Anyway, if that rooster egg was hatched by a toad or a snake, it would produce not a chicken but a cockatrice. It’s some kind of weird chicken-snake hybrid.
This fearsome monster could kill you with a glance, or a breath. The only way to kill it: take the wind egg and throw it over your house. If it gets to the other side and smashes, you’re safe. If it doesn’t make it all the way, I guess you have a cockatrice on the roof now?