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Why are Basilisks so rare if they are born from such common conditions? (A Chicken egg and a Toad sitting on top whilst hatching) A simple charm should keep the Toad stationary you'd have thought?

A chicken and a toad are all that are mentioned from what I can find. I understand the mythology is borrowed from the Cockatrice, but if this is genuinely the way these serpents are born then surely these creatures would litter the Wizarding world? I imagine these would also be quite common naturally if a toad happened to hop on an egg in a farmhouse.

Also Hermione finds this in the library, I assume not from the restricted section since only the older students studying Defence against the dark arts can use this without a note from a Teacher. So technically any student in the school could walk into the library and create one of the most feared creatures known to Wizard-kind. This doesn't seem right?

Some extracts below for context.

"This snake, which may reach gigantic size, and live many hundreds of years, is born from a chicken's egg, hatched beneath a toad."

"It is a creature bred by Dark Wizards. Herpo the Foul was the first to breed a Basilisk; he accomplished this by hatching a chicken egg beneath a toad which resulted in the creature known as a Basilisk. Basilisk breeding was banned in Medieval times. The practise can be hidden when the Department for Regulation and Control of Magical Creatures comes to check by simply removing the egg from the toad."

marked as duplicate by Skooba, Möoz, ibid, Valorum harry-potter Jan 24 '17 at 21:46

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    scifi.stackexchange.com/q/96872/31051 addresses why more people don't intentionally create them; not sure there's going to be an answer for why more aren't created accidentally, aside from how unlikely it is that a frog would incubate an egg to hatching – Jason Baker Jan 24 '17 at 21:41
  • As a side note: even if there was a possibility that there'd be naturally occurring Basilisks, it still wouldn't mean that they'd litter the place. Remember, Basilisks have a natural enemy, the spiders (and we know that there are plenty of those). – Möoz Jan 24 '17 at 21:49