mcbmagic is a lucky bastard, the Cobra Dragon and the Rattlyr Dragon are not the same creature, they're similar to an extent, as they both have hoods, both lack spellcasting, and both breathe fire, but there are differences, the cobra dragon for example has a poisonous bite.
IMHO, to get the stats for the Cobra dragon just add the following to the Rattlyr:
Augmented Criticals (Ex): A cobra dragons natural weapons have a critical range of 18-20.
Poison (Ex): Bite, DC equal to breathweapon DC, Initial and secondary damage according to table below:
Age Category - Damage
Wyrmling - 1d4 Con
Very Young - 1d4 Con
Young - 1d6 Con
Juvenile - 1d6 Con
Young Adult - 1d8 Con
Adult - 1d8 Con
Mature Adult - 2d6 Con
Old - 2d6 Con
Very Old - 2d8 Con
Ancient - 2d8 Con
Wyrm - 3d6 Con
Great Wyrm - 3d6 Con
Also, drop the rattlyrs electricity resistance and add poison immunity. Then increase the CR for each age category by 1 and increase its SR by 3 per age category that has SR. Everything else remains the same.