A distant cousin to the true dragons, the wyvern is a huge flying lizard with a poisonous stinger in its tail.
A wyvern?s body is 15 feet long, and dark brown to gray; half that length is tail. Its wingspan is about 20 feet. A wyvern weighs about one ton.
Wyverns speak Draconic, but usually don?t bother with anything more elaborate than a loud hiss or a deep-throated growl much like that of a bull alligator.

Wyverns are rather stupid but always aggressive: They attack nearly anything that isn?t obviously more powerful than themselves. A wyvern dives from the air, snatching the opponent with its talons and stinging it to death. A wyvern can slash with its talons only when making a flyby attack.
Improved Grab (Ex): To use this ability, a wyvern must hit with its talons. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and stings.
Poison (Ex): Injury, Fortitude DC 17, initial and secondary damage 2d6 Con. The save DC is Consitution-based.
Skills: Wyverns have a +3 racial bonus on Spot checks.
SEE WIKIPEDIA ENTRY: Wyvern

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System Reference Document -> List of Creatures (SRD)