Wyvern

Wyvern is a long, white chinese-dragon-like creature, which appears at certain heights. Unlike Harpy even 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.

The Wyvern only spawns once Hard Mode has been set.

Wyvern is shown in a dev blog post, where 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.