Massasauga

"ICE, BE WITH US"
- Massasauga, absolutely blasting the shit out of some Qulvo

Massasauga was a dragon from Ekellos, who at some point was taken over by the Tazza'Khool, although it isn't clear when this happened, or if it was even something which occured within the timeframe of the campaign.
He was only ever encountered twice: once, when the group encountered him battling a frozen lake full of Qulvo, and again as a member of Saan's emergency council.
Interestingly I think he's the only living dragon we've ever actually seen canonically.