Sorry, I kinda don't like the idea of your pets maybe dying :P
If I want my pets mauled by something guarding semi valuable items then I'll just go to neopet's snowager (no offense though)
Any way, dragon cave has all the eggs guarded by a mother dragon, but it seems to act more as a back story rather then affecting the actual rarity of the dragons themselves (could be wrong in some cases about the rarity thing, but they are all guarded and are common and rare alike)
It would be one particular creatu and perhaps its back story could be highly protective or highly aggressive