Tiamat

Tiamat , p.n., D&Dism 1. in ancient Chaldean, Babylonian, and Hebraic mythology, a sea-goddess central to the creation myth, who takes the form of a four-legged, scaly winged dragon. 2. from the AD&D Monster Manual, the extremely powerful queen of the evil dragons. [MC]
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Addendum: As the most powerful monster officially published in the early years of D&D, Tiamat was often used as a meter of PC power, with power gamers figuring out how quickly their characters could kill her & how little damage they would take in return. [MC]
Addendum 2: Sometimes still done today, though the terrasque has replaced the Queen in this role in most modern Monster Manuals since 3.0.