The original creations are unlocked by collecting various combinations of fiends (e.g. one of every fiend). Hitting for 99 999 with quick hits is a necessity at this point as they are the hardest creations you will find in the monster arena, but are still a hell of a lot easier than the dark aeons.
Each of the below fiends will cost 15 000 gil to fight with the exception of Nemesis which costs 25 000 gil.
Earth Eater
Greater Sphere
Catastrophe
Th'uban
Neslug
This battle is even more tedious than those with Jumbo Flan and Earth Eater with the sole redemption being that you only have to do it once. This newly designed fiend (the only one in all the monster arena) has a massive 4 million HP and 3 distinct stages. It can attack (sit on) a single character for a most likely fatal amount of damage, use Megaton which inflicts a large amount of damage and confusion, or worst of all slime which can hit one person or the whole party for a fairly large amount of damage and a variety of fruity status effects (armour and mental break, curse, poison and delay). It can also cast Curaga and Regen. If you are having trouble with slime, it isn't difficult to silence Neslug which pretty much restricts it to using Megaton or his normal attack.
At first just use quick hits until you've done 2 million damage and Neslug retreats into his shell (you should be able to do this before he even gets a go, if your agility is over 150 or so and you have auto-haste). At this point Neslug will no longer attack you, instead using Curaga on himself for around 8 000. Far worse than this he will regenerate upwards of 15 000 HP every single turn, and becomes immune to physical attacks. He will only come out of his shell if he replenishes the 2 million you've already taken off, or you take off a further 1 million HP which shatters his shell.
You need to ensure you do a lot more damage than the HP it heals every turn, and summoning an aeon is not wise either as he seems to get his HP back 10 times as fast (over 150 000 a turn!) if you do this. The only way to inflict 1 million damage in the shortest possible time period therefore, is to doublecast Ultima and then copycat it. At around 160 magic each Ultima will inflict about 50 000 damage, so against his healing this means you will take off around 80 000 a turn. Doublecasting Ultima takes forever, and gets very old after seeing it the 30 or so times you need to before moving Neslug to his final stage (this will take well over 5 minutes).
After this extremely tedious process, Neslug's shell will finally shatter and you can use quick hits to knock off the final 1 million HP and squash this seriously overgrown mollusc.