As if being a sea serpent big enough to encircle the world wasn't scary enough, Jormunqand the Midgard Serpent of Norse Mythology also has poisonous breath. In the Final Battle of Ragnarok, Thor breathes in too much of Jormunqand's breath while beating it to death with Mjolnir. He's Badass enough to try and keep going, but only manages to take nine steps before finally dying.
The Dragon of Beowulf is the fairly standard fire-breathing, treasure-hoarding, cave-dwelling kind. It probably eats people, too, but we never see it.
Revelation 19:15 "And out of his mouth goes forth a sharp sword, that with it he should smite the nations..."
A common version of the mythological beast catoblepas describes it as having poisonous breath (one description of the poisonous exhalation, curiously, matches the properties of the now-well known toxic gas phosgene).