arwink
Clockwork Golem
or you can plot-weave 
The body is originally picked up by a necromancer/evil cleric, who figures that the preserved corpse of a dragon is just the ticket for his next undead servant. Using his magic, he brings the corpse back as some low-key but powerrful undead (a zombie, ghoul or mummy, depending on the power level of the PCs), continually binding new magic into the creature to make it more dangerous. Given the preservation of the corpse, it still looks real enough that people may mistake it for a living dragon with strange new powers. It becomes a part of the necromancers neferious plots, and rumors abound of this strange new dragon that has returned from the dead.
At the same time, the spirit of the dragon is left wandering the afterlife. You don't necessarily see a lot of animated dragon corpses, largely because the God of Dragons (or some similar deity) is afronted by the raising and molesting of his children's corporeal remains. Aroused to anger at both the Necromancer and the Adventuring mercenaries that allowed this blasphemy to take place, the God grants the dragon's spirit a chance at repay the insult being made to his remains. Slap the feindish template on the original dragon, followed by the ghost template, and send him back to the material plane as a spirit of vengance.
The dragon spirit starts interfering with the necromancers plans, aided by the servants of the dragon god. the necromancer is peterbed, and starts looking for reasons why this incorporeal spirit is after him.
Once he works it out, he uses standard necromancer logic to place the blame on the PC's who sold the corpse to him in the first place. He starts laying bait for the dragon spirit, leading him towards the Pc's. At the same time, he uses the animated corpse to lure the PC's towards the spirit.
In the end, they're trapped between two opposing forces, both of whom blame them for the current problems.
If they defeat the dragon's spirit, then perhaps the necromancer tries to trap it before it returns to the afterlife - giving him more power. If they defeat the necromancer, perhaps the dragon spirit has the chance to possess his animated body, gaining some semblance of life with much more power.

The body is originally picked up by a necromancer/evil cleric, who figures that the preserved corpse of a dragon is just the ticket for his next undead servant. Using his magic, he brings the corpse back as some low-key but powerrful undead (a zombie, ghoul or mummy, depending on the power level of the PCs), continually binding new magic into the creature to make it more dangerous. Given the preservation of the corpse, it still looks real enough that people may mistake it for a living dragon with strange new powers. It becomes a part of the necromancers neferious plots, and rumors abound of this strange new dragon that has returned from the dead.
At the same time, the spirit of the dragon is left wandering the afterlife. You don't necessarily see a lot of animated dragon corpses, largely because the God of Dragons (or some similar deity) is afronted by the raising and molesting of his children's corporeal remains. Aroused to anger at both the Necromancer and the Adventuring mercenaries that allowed this blasphemy to take place, the God grants the dragon's spirit a chance at repay the insult being made to his remains. Slap the feindish template on the original dragon, followed by the ghost template, and send him back to the material plane as a spirit of vengance.
The dragon spirit starts interfering with the necromancers plans, aided by the servants of the dragon god. the necromancer is peterbed, and starts looking for reasons why this incorporeal spirit is after him.
Once he works it out, he uses standard necromancer logic to place the blame on the PC's who sold the corpse to him in the first place. He starts laying bait for the dragon spirit, leading him towards the Pc's. At the same time, he uses the animated corpse to lure the PC's towards the spirit.
In the end, they're trapped between two opposing forces, both of whom blame them for the current problems.
If they defeat the dragon's spirit, then perhaps the necromancer tries to trap it before it returns to the afterlife - giving him more power. If they defeat the necromancer, perhaps the dragon spirit has the chance to possess his animated body, gaining some semblance of life with much more power.