Final Fantasy IX has numerous enemy types, which affect how Killer abilities damage their targets, among other things. Humanoids cannot be eaten, Undead are hurt by curatives and fully healed by the spell Death (but not by LV5 Death or Iai Strike), Stone types can be instantly killed with a Soft (but not with Stona), slaying dragon types boosts Freya's Dragon's Crest attack, and Aerial enemies are immune to ground-based attacks. Heavy types (usually bosses) are immune to Instant Death and cannot be eaten, and Scan and fractional damage attacks always miss.
Some enemies, like Malboro, don't belong in any enemy class.
List of enemies
The humanoid class is mainly for special enemies and bosses and storyline characters fought as bosses tend to belong in this class, meaning they also belong in the 'heavy' class. The Alexandrian soldiers and Black Mage soldiers are humanoid, as well as the mirror versions of party members fought in Oeilvert. Man Eater makes normal attacks deal x1.5 damage to humanoids. Humanoids cannot be eaten with Quina's special abilities.
The class includes canines and other more bestial enemies. This group doesn't contain any bosses, apart from the Zaghnol that could be considered a boss for the Festival of the Hunt. The beast class is in a way a miscellaneous class where enemies that belong in no other class go to, as only Torama (demon) and Zaghnol (heavy) belong in another class as well. The Beast Killer allows the equipper to deal x1.5 damage to beasts with normal attacks.
The demon class includes many magical enemies, many of them bosses. Many enemies that derive from real world mythologies belong to the demon class. Many are also in the aerial or heavy classes, although the demon class overlaps with all other enemy types apart from stone and dragon. The Demon Killer support ability has the wielder deal x1.5 damage to demons with normal attacks.
The dragon class includes all big and small dragons, some of them bosses. Some of the strongest normal enemies belong in this class, like the Grand Dragon. Some of the dragons are aerial and the boss types are heavy, and one is an undead. Killing dragon type enemies powers up Freya's Dragon's Crest ability, and 100 kills is required to max it out at 9999 damage. Dragon Killer makes normal attacks deal x1.5 damage to dragons, but only Freya can learn it.
Undead class includes all zombie type enemies. One of them is a boss: the Soulcage. Undead take damage from curatives and can be instantly killed by revival spells. Undead Killer makes the equipper's normal attacks deal x1.5 damage to undead targets. Phoenix Down and Phoenix Pinion either leave an undead with single-digit health, or outright kill it (10% chance). Death restores undead enemies to full health, but Iai Strike and Doom and other instant death moves still work on them.
The stone class is the smallest class with only three enemies, none of them bosses. Stone types use petrifying attacks and can be instantly killed by using a Soft on them. Stone Killer makes the wielder's normal attacks deal x1.5 damage to stone enemies.
Insectoid enemies fall into this class and there is only one boss, the Deathguise. Many of them are also aerial, and one (Deathguise) is a demon. Bug Killer makes the wielder's normal attacks deal x1.5 damage to insects.
Flying enemies are classified as aerial, and this group is numerous. Aerial enemies are immune to ground attacks but tend to be weak against Wind. Many bosses are aerial. Despite its ambiguous name, the Bird Killer makes normal attacks deal x1.5 damage to all aerial enemies, not just enemies that are actually birds.
Heavy enemies are usually bosses. Heavy types are immune to Instant Death and cannot be eaten, and Scan and fractional damage attacks always miss. Friendly monsters are heavy. There is no killer ability to boost damage against them.