Wyvern

Wyvern is a long, white chinese-dragon-like creature, which appears at certain heights at Hardmode. Unlike Harpy, they can even spawn directly above the start point of a map.

The Wyvern attacks like a Devourer or Bone Serpent, making long, sweeping passes. Fighting them at high altitude and taking advantage of the lower gravity is useful for dodging, since they can do a lot of damage quite quickly. Weapons or Spells that strike a target multiple times (ex; Demon Scythe) are your best bet for fighting the Wyverns, as with any other worm.

Wyvern is shown in a dev blog post, where it appeared as non-hostile creature, and in the 1.1 trailer.

Fighting the Wyvern
One approach that seems to work well when fighting this monster ( especially on Large-sized maps ) is to let one spawn and then fall downwards without moving to the left or right. The Wyvern will follow you, ending up in a perfectly vertical angle. You could then attack it with any high-damage, high-speed weapon, such as the Minishark, Muramasa or Megashark.

When fighting the Wyvern for the first time, it is recommended to use Rocket Boots in order to change altitude quickly to avoid its sweeping passes. Featherfall Potions could optionally be used to help avoid contact with any part of the body as it passes. While fighting, wait until the wyvern is visible flying at you, then use the Rocket Boots quickly to get as high as possible in the air, avoiding the pass. Having Featherfall on would make avoiding contact with the body easier. Once in the air and above the sweeping wyvern, blast away at it with your weapon of choice (Clockwork Assault Rifle or Shotgun recommended) until it passes you completely. Then allow yourself to land and repeat until the wyvern is dead.

Once the player has access to the Angel Wings or Demon Wings, simply repeat the process above, but using the wings to stay in the air easily. This makes fighting the wyvern far easier.