|Just a Guess
I have no link but I would take a look at existing dragons, at traits and feats and give the dragon I want to customize some extras from what I find. Depending on what power level I aim for I might to take other abilities away.
Hissing green acid drips from the fanged maw of this black-scaled, horned dragon.
So black dragons are described as horned but the do not have a gore attack. Perhaps this one individual has lost part of his tail in a fight centuries gone but instead learned to use his horns for a gore attack.
Or you could give him the noxious bite, foreshadowing it by describing how the acid from this particular beast smells unbearably.
Or you take some dragon and give him both the stealthy and the nightstalker trait, suddenly you have a sneaky beast that can hide among his hoard despite his size. This could be folklore, that there is a dragon that appears from nowhere. Perhaps the party thinks he is using invisibility and prepares for it only to be surprised by him just using stealth.
I like giving them class levels. Sorcerer levels stack with their CL, allowing them to progress further without having to add a lot of racial HD and increasing the rest of the stats.
Sage bloodlines and Empyreal bloodlines allow you to swap your casting stat, so you can build a dragon off INT or WIS and have that factor into their personalities.