...rooster's egg hatched un... Rooster's Egg!? roosters don't lay eggs, and I won't fix it for you.~~Tomana from elsewhere
Well, actually, the Nethack Bestiary, which is referred to in the article (and which, by the way, is not at all a medieval account, but the inline help file of an ancient and quite famous computer game), quoting “Mythical Beasts” from Deirdre Headon , tells that :
- “Once in a great while, when the positions of the stars are just right, a seven-year-old rooster will lay an egg. Then, along will come a snake, to coil around the egg, or a toad, to squat upon the egg, keeping it warm and helping it to hatch”. 
So, “Rooster’s egg” is fine, even if it makes no more sense in real life than having a cold-blooded animal like a snake or a toad keeping the egg warm.
How it is related to the Harry Potter universe, however, is another matter. 184.108.40.206 09:33, November 20, 2011 (UTC)