Somewhat homebrewy here. Draconomicon has the feat, Dragon Cohort. It lets you pick a dragon cohort and treat its level as 3 levels lower. Your ECL is 15, so normally you could only get a cohort of max level 13. With Dragon Cohort you get one of max level 16 as long as its a dragon.
From the Draconomicon there is the Abyssal Drake. It's Huge, so you can ride on it, and it has fire immunity. It's ECL is 15, so if you can get your leadership score high, you can pick it as a cohort. It can also fly, so that covers nedz point. Only problem is that it is technically an outsider, not a dragon from the specific list of dragon cohorts. Thus, you'd need DM approval. I think its balanced though.
Other obvious problem with the Abyssal Drake is that it is Chaotic Evil
EDIT: You could also slap a Fiendish template onto a Rhino or an elephant. That gives it fire resistance. We've also got Pyrohydras. I'm sure you can convince someone in your group to dominate the hydra to serve as your mount. You could also use Handle Animal on the Hydra to train it. DC 32 to train a 12 headed one.