Neslug is an optional enemy in Final Fantasy X, bred in the Original section of the Monster Arena after at least one of each fiend roaming throughout Spira is captured. The design for Neslug, a hideously mutated snail, is only found here; hence making it the only true "Original" monster.
Neslug will attack with Megaton, which causes Confuse. Its Slime attack will hit all party members, inflicting Armor Break (always), Mental Break (always), Poison, Curse, Silence (1 turn), Slow (1 turn) and Delay (strong). Also, the Armor and Mental Breaks that come with Slime happen to ignore immunity, leaving Aeons more susceptible to damage. After losing 2,000,001 HP, Neslug will retreat into its shell and initiate Auto Regen regenerating massive amounts of HP, and is immune to all physical attacks. After losing 3,000,001 HP, its shell is broken and Neslug emerges.
It drops weapons with Piercing (only for Auron and Kimahri) and Triple AP, and armor with HP +% abilities, HP +30% being fixed.
When Neslug retreats into its shell, the player should not summonaeons as it will make Neslug start regenerating over 100,000 HP per turn. During this time Neslug is immune to physical attacks, making Ultima handy. Spare Change also works against Neslug but requires over 100,000 gil to inflict any notable damage. When Neslug is down to 1,000,000 HP its shell will break and disappear. Wakka's Attack ReelsOverdrive is a useful way to ensure victory.
It is worth noting here that in Final Fantasy X, "physical" and "based on the Strength stat" are not always the same thing, and specifically, that overdrives never count as physical damage. For this reason, Attack Reels and Blitz Ace (among others) will work on Neslug even when in the shell, and can be very useful to quickly break it.
The shell phase can be bypassed altogether if Neslug is reduced from over 2,000,000 HP to under 1,000,000 HP in a single attack. This can only be achieved with Attack Reels, Passado or Oblivion.
If the game's data is examined, Neslug's entry is not near the other Original Creations, but rather, near the fiends and bosses of Mushroom Rock Road and Djose. This may suggest that Neslug—presumably in a far weaker form—was originally going to be a boss, or perhaps even a random encounter, somewhere near Operation Mi'ihen.
Neslug and Espada are the only arena creations immune to Doom. They are also the only two arena creations with a Zanmato level of 4; the others all have a Zanmato level of 5. Neslug also breaks the pattern of Original Creations having the 6% critical hit bonus on their equipment drops.