After you had complete the game, Let the credits pass by and you can continue playing. Just complete this part and you get to see the bonus ending.
Care And Feeding Of Claws
Claws are by far the best way to get Feathers, you use them more and lose fewer HP. Save the Elemental Seeds (Wind, Fire, etc) for upgrading your Saddle. When you get claws like the "Flame" "Dwarf" and so forth, even if they are cursed or already +1 or something...un-curse them if you have to, and use a polish card(s) to get them at *least* to +1. Then use them until they break...more often than not you will get a Feather related to the effects of the Claws (Chaos will give you confusion, etc). Use this to help the Golems freshen up the tree in the Village since you can get feathers you lose over again easily.
Fight Black Goblin At The Beginning
When you get to the second floor of the first dungeon, you will see a goblin fighting a muz. It will win and turn into a dark goblin. Let Mog get beat up by it and it will turn into a black goblin. You can try to fight it. You need to be at least level 15 or more to win.
Get Rahmu Feather
In the restaraunt there is a drunk marlboro in the top left corner, keep on using thunder books on him until he turns back into Rahmu.
Get Sylph Feather
Go to the beach until you see a treasure chest, take it, keep going back until you see a treasure again, do this 3 times and Sylph will appear giving you the feather.
Get Titan Feather
Examine the statue in the center of town, after a few times of looking at it you should see Ben there, (a brown behemoth), he should ask you what the statue needs, if you answer by saying "an earthy smell" and come back in a while he'll ask you for a quake book. Come back in a bit and the statue will turn into Titan!
Kill Boss In Cid's Tower
First, try to level up your magic spells and try to get your feathers from the black magician. Also, get your feathers from the drunk Malboro. Titan from the statue and Sylph from the beach. Then, put the game on 2P from the options screen. As soon as you attack, make the Cidtank shoot the Imp Robot's.Try to not get hit by the bombs and if the Cidtank's life is low, use one of the feathers of Sylph, Titan, or Ramuh. You should be able to kill them easily.
Bonus Ending...