Wyvern

Wyvern is a long, white dragon-like creature, which appears to spawn in Floating Islands but also seems to spawn anywhere if a player is high enough, even directly above the start point of a map.

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

Trivia

 * Looks similar to Haku's dragon form in the animated movie Spirited Away.
 * It's a never ending story.