In my game... (my players please stay out):
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The PCs found three blue dragon eggs in Carceri when they killed an adult male Blue. They brought the eggs back to Earth, where they eventually hatched. Two of the PCs got baby wyrmlings, one went to the NPC apprentice of a high-level Evoker.
Dragons in my game have no automatic color-based alignment ... but these are actually half-fiend dragons, additionally with the Shadow Creature template. Their natural inclinations are strongly Evil.
However, they were born on Earth, drew their first breath in the Mortal sphere, and are therefore imbued with mortal souls and free will.
So far, one of the PCs has undertaken to make his little friend more moral and good (the PC is a Paladin), and it's working. The dragon is basically good.
The other PC (an Eldritch Knight) hasn't much cared, and his dragon (when left alone) went off and assumed leadership of a thieves' guild, then feigned being a hostage (turning on and killing his helpless "kidnappers") when the PCs came in to "rescue" him. He's got a high Bluff check.
Now Mom is back, and she has put out some contracts for the retrieval of her lost children. She has taken back the (mostly evil) NPC apprentice's dragon, and she will probably convince the Eldritch Knight's wyrmling to join her, if he doesn't take steps to "moralize" his charge. And having a former party member suddenly become an enemy will be VERY bad for them, since their foes will know ALL of their tricks and abilities.
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Cheers, -- N