From Lotro-Wiki.com

Difficulty Indicators

Name Color / Con Level

Creatures that you can fight have their names in specific colors to indicate their difficulty relative to your own level. This color coded designation is often referred to in the gaming community as con or con level (which is a shortened form of either constitution [health] or of consider, implying that the player must consider the creature's level before deciding whether or not to attack).[[1]] [[2]] The con level color will change as your character grows.

As an example, you will find that a level 15 bear appears purple and is practically impossible to defeat when your character is level 7. That bear will also be extremely aggressive toward you, and will offer a significant amount of Experience points, or XP. Once you have gained level 15 yourself, the bear's name will appear in white and be much less of a challenge to defeat; it will also offer a lesser amount of XP. When your character is level 30, the bear's name will appear in grey and will offer no challenge whatsoever; for this reason, you will no longer receive any XP for defeating this level 15 foe.

Incidentally, creature names of any level may also appear grey if they have already been attacked by another person. This condition is simple to identify because the creature's morale bar will also appear grey, rather than yellow or orange.

From easiest to hardest:

Grey - 9 or more levels below - yields no XP / Was attacked by another person.
Green - 6 - 8 levels below
Turquoise - 3 - 5 levels below
Blue - 1 - 2 levels below
White - (creature has the same level as yourself)
Yellow - 1 - 2 levels above
Orange - 3 - 4 levels above
Red - 5 - 7 levels above
Purple - 8 levels above (creature is far greater than yourself)

Grey creature.	Green creature.
Turquoise creature.	Blue creature.
White creature.	Yellow creature.

In general it is possible to defeat all "normal" creatures up to the difficulty level "orange", while "red" and "purple" enemies are impossible to kill alone, because they are too hard to damage.

Difficulty Rank

Creatures also have a colored ring surrounding the icon attached to their stat bar. The color of this ring indicates their status as a solo or fellowship creature, called "rank" by the developers.

Swarm
- green ring
- has decreased morale relative to its level and can be easily defeated one at a time, but in groups they may offer a greater challenge than a normal creature
Normal
- blue ring
- can be "soloed" (defeated by a character playing solo)
Signature (Special for creeps)
- red ring
- can be soloed, but will be challenging
Elite (Defender for creeps)
- glowing red ring
- may require several players to defeat
Elite Master (Master Defender for creeps)
- burning red ring with 4 spikes
- may require a full fellowship to defeat
Nemesis (Avenger for creeps)
- burning red ring with 6 spikes
- may require a raid of two or more fellowships to defeat)
Arch-nemesis (Hero for creeps)
- burning red ring with big flames around the eye
- may require a raid of three or four fellowships to defeat)

Swarm creature.
Normal creature.	Signature creature.
Elite creature.	Elite Master creature.
Nemesis creature.	Arch-nemesis creature.

A complete listing of creatures organized by difficulty rank can be found at the Category:Creature_difficulty page.

Creature Catalog

Most creatures in the world currently fall under one of 12 types. Many weapons and skills in the game only work, or do extra damage, to creatures of a specific type.

Ancient Evil: Ancient Evil are creatures that served Sauron's master prior to Morgoth's expulsion from the world. This category includes such creatures as the Morroval, Gauntmen and Balrogs.
Beasts: Beasts are the animal life you run into throughout the world such as wolves, chickens, wargs and other such creatures.
Creatures of Nature: Creatures of Nature are Ents, Huorn (living trees), Bog-lurkers and other living plant type creatures.
Dragon-kind: These range from the great Drakes through the lowly worms you will meet across the world. Actual dragons... There is 1 living in the world "somewhere". Seek and you may one day find it.
Dwarves: The Dourhand Dwarves are of dwarven stock but are evil by nature. As such there is a category of creature type that covers them.
Giant-kind: The great giants in the world are considered of their own type.
Man: Many evil men reside in the world and there are abilities and weapons that are specialized to deal with these evil men therefor they have their own category.
Objects: Covers siege engines in the Ram Dúath and Fumeroles in the Rift.
Orc-kind: Orc-kind runs the gamut from the lowly Goblins to the mighty Uruk-hai and from level 1 through level 53 bosses. Many weapons exist that provide advantages against the various species within this Genus.
Spiders and Insects: Insect life throughout middle-earth is abundant. There are insects throughout each and every non-"safe" zone in the game (Example: Bree-town has none). It runs from annoying swarms of Gnats to deadly Spiders of ancient lineage on a par with Shelob's age.
The Dead: The Dead, or undead if you prefer, are plentiful around different parts of the world ranging from Wights and spirits animating waters through crawling severed arms that will try to defeat you as you adventure through the world.
Troll-kind: Trolls come in many shapes and sizes and have their own properties and abilities.
Unseen: This special category covers the Nazgul and other special creatures closely related to them in one fashion or another.

Bestiary

Beasts Birds and mammals.	Spiders and Insects Eight-legged menaces.	Critters The harmless, idyllic animals...
Creatures of Nature Living Trees and Ent-wannabees.	Dragon-kind Fire and flame!

Humanoids

Dwarf (Mob) Dourhands and co.	Man (Mob) Not all Men are Free.	Giant-kind No one at home will believe this.
Orc-kind Servants of the Dark Lord.	Troll-kind All kind of trolling.	Ancient Evil Lords of the Dead.
The Dead Rotten flesh and bones.