Wyvern

Wyverns are large Hardmode enemies that spawn at high altitudes. They resemble oriental dragons, are long and white, and follow the Worm AI.

Wyverns spawn at any lateral map location, day or night, but only at Floating Island altitudes. You will know if the conditions are right for Wyvern spawning if you see Harpies, as both spawn under the same conditions, though Wyverns spawn much less often.

However, wyvern spawn rates seem to be dependent upon altitude. At the lower reaches that harpies are capable of spawning, (when it's still possible to get other types of enemies,) wyverns will be incredibly rare, and players might have to camp for game days to see one. At the maximum altitudes, where the sky fades away and gravity starts to affect players less, wyverns become significantly more common. Therefore, when hunting wyverns on purpose, one should try to climb up to the highest they can get to bait them.

Tips

 * Wyverns attack like Devourers and Bone Serpents, except they tend to swoop down from above, and they have no dependency upon blocks to maneuver, making them extremely nimble.
 * Fighting Wyverns can be difficult, as they deal respectable amounts of damage, are very fast, and continue homing in on the player almost until making contact, which makes dodging difficult. Try to time jumps so that you're falling when the Wyvern is close to making contact, as the speed of your fall will help you avoid contact. Equip all available jump-boosting Accessories. Gravitation Potion and Swiftness Potion will also help.
 * The best way to dodge the wyvern's swooping attack is to get it to attack you at around a 45-degree angle (run away from it until it's at around that angle,) then jump over its head as it swoops in. It reacts more slowly to swift vertical movement than horizontal movement.  Having multiple rows of platforms to jump to can help.
 * Pre-hardmode armored players will have extreme difficulty fighting a wyvern unless they set up an arena for doing so. It may be best to construct an arena for fighting them.  If the player has a large boss-fighting arena already, the player may just climb up above that arena to bait a wyvern into attacking, then let the wyvern chase them down into the arena for the actual fight.
 * The head hits for significantly more damage than the body - if you're about to be tangled up in the coils of the wyvern, it's better to be hit by the body segments than the head. With hardmode-tier armor, the body deals fairly trivial damage.
 * Heart Statues and, if you use magic, Star Statues make surviving this battle significantly easier, especially if you have a Cross Necklace. It then makes fighting the wyvern a matter of out-lasting it.
 * With the Gravitation Potion killing the Wyvern is relatively easy if you have a bow with Unholy arrows or Shuriken or any piercing effect projectile. Simply wait for him to come for you in the sky and fall to the ground. The Wyvern will follow you in a vertical posture, allowing you to damage him 5-6 times with the same projectile. Reverse the gravity when you see the ground and go wait for him back in the sky. Rinse and repeat.
 * As with any large enemy, weapons that pierce targets for multiple strikes (eg. Demon Scythe, Dao Of Pow, Spears) are more effective. Hardmode-tier area-of-effect attacks like the Nimbus Rod and Death Sickle are horrendously effective against wyverns, killing them in seconds.
 * Using the Inferno Fork or a mage item with either great damage or fast speed can easily defeat this mini-boss
 * With a long sky bridge and Hermes or Spectre Boots, running continuously in one direction can keep the Wyvern behind you for longer periods, allowing a long-range weapon to be spammed behind you. The Wyvern will still get in front of you often, but this method provides more solid attack time.
 * Using explosive ammo is useful too because of it's splash damage being able to damage multiple segments at once.

Trivia

 * Although the name "wyvern" refers to a western dragon depicted as having two wings and two hind legs, but no arms or forelegs, Terrarian Wyverns are shaped after Chinese dragons in Chinese folklore, called 'Lung Dragons' in Dungeons & Dragons.
 * Because of their color, body shape and general style, Wyverns bear a close resemblance to several dragons from popular culture, such as Haku's true form, from the animated film Spirited Away.