Elite Monsters and Elite Bosses have prefixes that give them a random effect. This thread is a guide that tells you which prefix does what.
Buffs:… (more) (less)Speedy - Increases Speed
Beefy - Increases Weapon Defense
Magic Resistant - Increases Magic Defense
Powerful - Increases Weapon Attack
Healer - Periodically recovers HP equal to the cube of the monster's level divided by 15 to user and nearby enemies
Solid - Temporarily immune to non-critical hits
Invincible - Temporary immunity
Reflective - Gives Damage Reflect that deals damage equal to the square of the monster's level divided by 7.5
Commanding - Creates a blue aura that grants the preceding prefix's buff when monsters are in the aura
Debuffs:Sticky - Causes a modified Slow which drops your speed to 50%
Fragile - Causes Weakness which prevents jumping
Shadowy - Causes Fear of Darkness which reduces your vision
Befuddling - Causes Confusion which reverses your movement keys
Magnetic - Causes Magnetized which draws you to the monster (Removed)
Gassy - Creates a Poison cloud that damages you when you are in the cloud
Poisonous - Causes Poison which deals damage over time
Evasive - Causes Blindness which makes you miss often if you are not a Magician
Seal Master - Causes Seal which disables your skills
Transformative (Renamed) Master of Disguise - Can transform you into a pig which will render you helpless
KO (Renamed) Overwhelming - Causes Stun which stops your movement and skills
Overwhelming (Renamed) Unstoppable - Causes Shadow of Darkness which deals damage if you do not move
Accursed - Causes Curse which reduces your EXP gain by 50%
Magic Eater - Dispels buffs from players (Removed)
Undead - Causes Undead which cuts your potion recovery by 50%
Dry - Causes Potion Lock which prevents potion usage
Petrifying - Causes Bind and you must press < and > alternately to break free otherwise you die
Seductive (Renamed) Charming (Renamed again) Charismatic - Causes Seduce which locks your control of your character
Debilitating - Creates a dark aura that causes the preceding prefix's debuff when you are in the aura
Black Chains - Adds a %HP attack that inflicts a random debuff