Final Fantasy X/Secrets and Sidequests/Monster Arena/Species Conquest
The species creations are unlocked by collecting a certain number (usually around 4) of every fiend in a species (e.g. all types of flan). Hitting for 99 999 with quick hits is a necessity for the harder species creations (as is having high agility and evade), but you might get away without it for some of the species creations with lower HP. Each creation embodies the theme for that type of monster, so will have at least one very large stat (e.g. Jumbo Flan has large magic and infinite defence stats). Most of the monsters here will also drop spheres for their stat when defeated, making the species creations essential for getting more powerful.
Also, Species Creations have much, much more health than Area Creations in general, so watch out.
Each of the below fiends will cost 8 000 gil to fight.
At 850 000 HP, Fenrir, the first of the Species Creations will be by far the most powerful wolf enemy you will ever face. He's fast, meaning even with 150+ Agility you will still only get less than six turns in between his attacks, although Hastega and Quick Hit will soon put him back down the turn bar where he belongs. Unfortunately, his Evasion stat is also pretty decent, so expect quite a few attacks to miss. His attacks consist of Fangs of Chaos, which does a large percentage of a character's current health to one character and inflicts Confusion, Fangs of Hell, which is instant death, and Fangs of Ruin which lower stats and does a lot of damage too. Attack Reels and Blitz Ace are the way to go here, as always, with Quick Hits in between. Fenrir drops Agility Spheres; 1 normally, 2 for an Overkill (this drop rate applies to all enemies which drop stat spheres).
Yojimbo can kill Fenrir in one summon, if you're lucky enough to keep him alive. If this quick method fails, just use the strategy above.
Ornitholestes is the souped-up lizard fiend, with 800 000 HP. You will notice that although Ornitholestes is slow he has a very high evade rate, so expect few attacks to actually connect. Keep trying to attack until it does enough damage to someone to knock them into Overdrive or it accumulates on its own, then Entrust it to either Tidus or, preferably, Wakka if it isn't them and use Blitz Ace/Attack Reels. Poison Touch does a proportion of a character's maximum HP to them and inflicts Poison (this is a 100% chance without Poisonproof or Ribbon, and if you notice that's enough for one turn of Poison to kill the afflicted character), and Drain Touch is pretty similar but will heal the damage dealt from Ornitholestes. He also drains fire attacks.
Pteryx is the small bird species creation, and has only 100 000 HP which means two hits at anything more than 50 000 a hit will kill it. However, its high evade rate will mean most attacks miss. You've got two choices here; Doublecasted Ultima (which even for a 130+ Magic stat will hit for 30 000+) or an Overdrive - either Blitz Ace or a perfect Attack Reels is a guaranteed Overkill. Pteryx's only attack of note is Beak of Woe which is about 2,000 damage to everyone, plus Curse. If this happens to throw a spanner in your works just use a Holy Water to clear it up. Pteryx drops Evasion Spheres.
The greatest of the flying insects, Hornet has 620 000 HP and is incredibly accurate regardless of your Evasion stat. He has a poison-inflicting sting attack and a death-inflicting sting attack, neither of which ever miss (you can only avoid the status effect with either Poisonproof/Ribbon and Deathproof respectively), and his normal sting attack which inflicts a whole host of different statuses including death (but it might miss). Once again, Quick Hit and Attack Reels should swat this insect down for good. He has a fairly high evade rate compared with most monsters (although nowhere near that of Fenrir/Ornitholestes/Pteryx/Vidatu). Hornet drops Accuracy Spheres.
Vidatu has only 95 000 HP and low Defence so land one hit at 99 999 and you win. The difficulty is landing that one hit, as Vidatu is another example of a high evade-rate monster. Repeatedly Quick Hit it until an attack connects or just use an Overdrive. His relatively weak collection of magic (including Ultima) will barely tickle you.
One-Eye is the ultimate flying eyeball with 150 000 HP. However, in terms of attacks he's pretty brutal. If he gets the first turn it will always be Shock Wave, which is potentially upwards of 9999 to everyone plus a truckload of status effects including confusion. Black Stare is his other trick, which is instant death to one character. He also has a decent evade rate (similar to Hornet). At low-levels beat him down with Aeons as One-Eye's attacks are pretty useless against them or use Attack Reels - providing you can hit for 9999 a hit a perfect Attack Reels will kill it. One-Eye drops Triple AP weapons on occasion, but his normal drop is Magic Defence Spheres.
 Jumbo Flan
Jumbo Flan is by far and away one of the most annoying of all the creations in any category. At 1 300 000 HP this doesn't seem like much of a problem besides a lot of health but when you factor in the following...
- He is in constant Reflect status.
- He has maximum Defence and a pretty high Magic Defence.
- He is completely immune to physical attacks.
...you begin to have a problem. His absurdly high Defence means Attack Reels and Blitz Ace won't make even a tiny dent in his large HP total, with Attack Reels reduced to around 3000 damage a hit for a Strength of around 150. So, how do you beat it?
 With Anima
Anima has the special attack 'Pain' which is unique in that it is unaffected by any defence stat. Thus, a fairly powerful Yuna will be able to allow Anima to inflict upwards of 50000 damage a hit to Jumbo Flan with Pain (sadly Jumbo Flan is immune to instant death). However, this is not enough to take it down, so teach it Lancet, Haste, Dispel, Slow, Reflect, Shell and Regen. Start the battle by casting all these white magic boosts, then repeatedly Pain Jumbo Flan, using Lancet to heal where necessary.
 Without Anima
Cast Reflect on one of the members of your party. Cast all your spells on the character with reflect. If the character with reflect wants to cast drain or any cure spells cast it on the Jumbo Flan. Doublecast helps. You can also use Ultima since it penetrates Reflect, but it has a long animation. Use dispel if he casts regen on himself. Keep casting spells and he will eventually fall.
 Nega Elemental
Nega Elemental is very easy to defeat; just use Dispel on it when it attempts to use Reflect. Keep on using double-cast Ultima or Flare. All other physical attacks lead to its Ultima attack, so be prepared with Auto Phoenix or Auto Life.
A large armoured mole, you obtain this by catching at least 5 of each "large armoured mole" species throughout Spira. With this monster you will need a large amount of strength and defence, or just defence and a reasonable amount of strength, and a weapon with piercing capabilities.
This armored dragon is one of the toughest opponents in the monster arena with his very high damage and triple attack. Fafnir can use all elemental attacks and they do a decent bit of damage if your characters aren't high enough in defense and magic defense. Like all armored dragons in the game, it has high defense, which takes reduced damage from all weapons except piercing. Make sure your party has celestial weapons and auto phoenix. He normally drops 20 light curtains and 40 for an over kill.
When battling him, your best party would be Tidus, Auron, and Wakka. Use Blitz Ace and perfect attack reels. Also, if Yuna is strong enough, you could use aeons, as you would have done against the Magus Sisters or Anima.
 Sleep Sprout
 Bomb King
This is the easiest battle of all the creations/original. If you can hit all 9's on him (99,999), then you're set. He doesn't counter anything, and it's really easy to hit him. 5 hits and you take the throne.
This is the only "Bomb" type enemy who doesn't blow after 3 hits. But he grows on every hit, and his attacks will getting stronger too. When he's huge, he will always attack with "ultima", and that hurts. Five 99999 attacks on it, and you will see the pyreflies.
An easy method is to equip someone with fire eater, and counter magic. Damage him until he swells for the first time and starts using Fira. Then have anyone that doesn't have these 2 abilities escape, and throw a hast on the Bomb King, then continue to simply defend on your turn. Bomb King will use Fira on you, healing damage because of Fire Eater, and then you counter, and counter attacks do not cause him to swell. Using this method, it is impossible to lose.
His first move always is "Charging", followed by "Salvo", a fire attack to all of your characters. So, when you see "charging", use "nulblaze". He's immune to all magics, so this battle is all strenght. He drops Strenght Spheres. Two spheres for an overkill.