My group betrayed a goblin barge captain that helped get them from Elturel to Baldur's Gate...
The captain had a typical minor reason to go to Baldur's Gate. He was taking goods made in the farming regions to the south and delivering them to BG. In doing so, he used the last of his savings to buy the goods in an effort to make a minor profit when he arrived. No insurance, of course. During the travel, it went extremely smoothly and the group even made friends with him. Upon arriving he mentioned that the priority was dropping off the livestock as they aren't allowed deeper in the city. The goblin mentions that he is worried about his merchandise on the vessel, so the group offers to take the livestock to the butcher and recieve payment for him. Well, they take the livestock and do neither. They unleash them in the streets and docks of BG. Effectively, ruining this goblin.
Now, they continued their journey and are doing a side quest in the city. While this is occuring, I'd love for the goblin to look for a way to make some money. So, there happens to be this Dragon Cult moving a hoard of gold through the city. I'm having them hire on this goblin. In his hatred of the group, I want him to offer himself up to one of their transformative rituals. What would be the best creature/dragon hybrid to turn this guy into? I'm looking for something that would give a group of six level 5 or 6 (it'll be a bit until they run into him) players a tough time.