Bukkit

Watch out, pigs will fly after you install this plugin. At spawn, a configurable percentage of mobs will take off and fly around. The effect works very well for chickens, and is pretty funny with pigs. A saddled flying pig can even be ridden, if you can catch it. Other mobs can fly too, if configured, although your mileage may vary. Watch out for flying ninja creepers.

Features:

• Flying mobs
  
• Separate configs per world
  
• Configurable wand to make a mob fly ("pigasus.wand" permission required)
  
• Per mob type wand support, using "pigasus.wand.<mobtype>" permission node

This video from OfficialK4G does a great job of showing off how mobs fly with Pigasus. Keep in mind that they have turned everything up to eleven for this video. :) Thank you OfficialK4G!

Installation:
Drop the Pigasus.jar file into your Bukkit plugins directory and restart/reload the server. A default config.yml file will be created in the plugins/Pigasus folder, with all tweaks disabled by default. Update the config.yml file and restart/reload your server to activate the tweaks.

Config file:

• WandItem: Right-click a mob holding this item to cause it to fly
  
• Worlds: This section define which worlds can have flying mobs, which types are allowed, and what the likelihood is that a flying mob will spawn. Here is the default config for this section:
  

  Worlds:
      world:
          Pig: 10
          Chicken: 25
  

  This means that in the world named "world", flying pigs will have a 10% chance of spawning, and flying chickens will have a 25% chance. No other mobs will fly.

Supported flying mobs: Pig, Chicken, Cow, Sheep, Wolf, Squid, Zombie, Skeleton, Spider, Creeper, Slime, PigZombie (use these names in the config file) Let me know if you think other mobs should be allowed to fly.

Changelog:
Version 0.2.2 (released 9/29/2012)

• Added "pigasus.wand.<mobtype>" permission support

Version 0.2.1 (released 9/16/2012)

• Mob flying height randomized a bit
  
• Works better with worlds surrounded with the void (like The End)

Version 0.2

• Updated to support new Bukkit events system (MC1.2.3 compatible)

Version 0.1

• Initial release