Boss Monsters

Difficulty:  Depends on the monster
Level Restriction:  None
Timer:  None
Can be Solo'd?  Yes
Rewards:  Various, also depending on the monster

There are a whole lot of boss monsters out there, most of which carry unique items with them.  Often times, these bosses will drop their special items and more often than not, they will also drop whatever trophy that class of monster carries (eg, Motes, Skins, etc).  The trick is finding these guys!  They are all pretty rare, but no doubt you will come across plenty of these guys in your career.

Aun Ralirea - Aun Tumerok boss.  Drops a Clay Figurine worth a one-time skill credit and Ebon Spine Harpoons, hollow cold spears and thrown weapons.

Basalt Golem - Obsidian Golem boss.  Drops the Basalt Blade.  That weapon can be converted into the Obsidian Axe, a flaming axe.

Canescent Mattekars - Large Mattekars.  Drop Canescent pelts which can be crafted into a special robe, by adding Grievver Acid or Olthoi Ichor.

Cold One - Sclavus Boss.  Drops the Serpent's Fang, a snake-shaped acid spear.

Crystal Lord - Crystal Golem boss, surrounded by dozens of Zefir-like minions.  Very dangerous, lots of health, and drops a ton of high-quality loot.

Dark Inferno - Shadow boss.  Drops the Scroll of Dark Rain, a scroll that can be used to add a one-time 3 point bonus to Magic Defense.

Dark Sorcerer - Dark Magus boss.  Drops the Dark Sorcerer's Phylactery, a spellcasting orb.

Dark Myrmidon - Relic Bone boss.  Drops the Osseous Mace, a flaming leg bone from a skeleton.

Diamond Lord - Diamond Golem boss.  Drops the Diamond shield, a very nice shield with good AL and spells.

Hea Tuperea - A Hea Tumerok hunter and his pack.  He drops a broken Crop (a slashing Staff weapon) and his Reedsharks drop special collars, which raise your strength, endurance, coordination, or quickness and Implants, special casting orbs that can cripple a target's Coordination and Quickness.

Hoary Mattekar - Mattekar boss.  Drops a Hoary pelt, which can be traded in for the Hoary Mattekar Robe, a classic robe with armor level.

Lord of Decay - Skeleton boss.  Drops the Skeletal Atlatl, a lower level Atlatl made from arm bones of a skeleton.

Lugian Warlord - Lugian boss.  Drops the Soul Staff, an electrical staff with faces of the Developers on it!

Master of the Pack - Reedshark boss.  Drops the Fetid Dirk, a low-level acid Dirk.

The Pure One - Doll boss.  Drops a purple crown that is far more stylish than it is functional.

Sentient Fragment - Dual Fragment boss.  Drops the Crystal Sword, a higher damage Slash/Pierce sword.

Swamp King - Mosswart boss.  Drops the Gauntlets of Marksmanship, a pair of gauntlets that improve missile skills.

Tundra Mattekar - Mattekar boss.  Drops the Robe of the Tundra, a decent robe with armor level.

Singularity Caul Bosses

There are also bosses that only appear on the Caul.  All of these drop rewards on occasion.  Unfortunately, each piece has a wield requirement of 100+ on it.

Atakir, the Dark Prince - Zefir boss.  Drops Atakir's Blade, an imbued multi-strike dagger.

Befouled Doll - Doll boss.  Drops a Doll's Eye, a ring

Colossal Mite - Mite boss.  Drops a Mite Leg Bone, which can be crafted into a high damage imbued Mace.

Doomshark - Reedshark boss.  Drops a hide that can be crafted into the Doomshark Hide Coat.

Gikar - Margul boss.  Drops Gikar's Dream, a high damage staff.

Listris Nefane - Niffis boss.  Drops Nefane shells, which can be be crafted into Nefane shield that can be stamped.

Ravager - Carenzi boss.  Drops his Fangs, which can be crafted into Needletooth, a high damage imbued Katar.

Parfal Nefane - Nefane boss.  Drops Nefane pearls, spellcasting orbs that can also be turned in for Writs.

Plaguefang - Reedshark boss.  Drops his hide, which can be made into a high AL robe.

The Puppeteer - Marionette boss.  Drops his skull, which can be made into a spellcasting orb with a special war spell.

Scourge - Rat boss.  Drops a hide which can be made into high AL leather leggings.

Snarl - Carenzi boss.  Drops his hide, which can be made into a Jerekin that has special defensive spells.

Banished and Fallen Monsters

During the Infiltration patch, two new categories of monsters appeared - Fallen and Banished Monsters.  Fallen Monsters are Valley of Death and Singularity Caul variants that are found in those two locations.  Banished Monsters spawn in the 40-60 areas.  Each creature drops a specific weapon.  Banished monsters drop 300/270 wield weapons with high damage, BD VI, and a minor aptitude.  Fallen weapons are 370/335/330 wield weapons with high damage, crushing blow and biting strike (no biting strike for the orb), BD VII, and a moderate aptitude.  Fallen Monsters also drop a portal gem to Sezzherei's Lair.  Sezzherei is a Margul who drops a 'Sezzherei Slayer' title token and a Salvager's Helm - an AL 10 helm that casts minor Salvaging aptitude if you have 150 base salvaging. 

What To Do With Your Weapons

If you don't want the weapons you find on Rare, Fallen, or Banished monsters, there's a collector in Stonehold who can help you out.  Look for Belinda du Loc.  In addition to giving you a kill task for Rare, Fallen, and Banished monsters, she will accept Banished and Fallen weapons for XP (a percentage reward of about 2.5% capping at 500k and 3.5M respectively).  She can also upgrade many of the rare monsters' trophies to make them more useful.  If you returned the resulting Enhanced items to her, she will give you 500k XP as well.