Who was the first dragon in D&D?

Bahamut The character appears in the first edition Monster Manual (1977). In this book, the Platinum Dragon was given the personal name Bahamut. He was also identified as a lesser god for dragons in the book Deities & Demigods (1980).

https://en.wikipedia.org/wiki/Bahamut_(Dungeons_%26_Dragons)#:~:text=The%20character%20appears%20in%20the,Deities%20%26%20Demigods%20(1980).