Here is a list of all(or most) of the bosses in game and they're spawn timers. Feel free to correct any of the timers if they are mistaken or have changed and any bosses not on this list.
Alton Carbon Zule - 10 hours
Anbrasxas - 10 hours
Andreev the Quick - 1 hour(Possible spawn holder)
Arllandah - 10 hours
Artraxis the Elder - 10 hours
Asterion Mino - Triggered by Minotaur Elites
Athropiniax - 8 hours(Possible spawn holder)
Black Tail - 10 hours
Bonesnarl - 1 hour
Boss Scalo - 4 hours
Brood Queen - 2 hours
Captain of the Guard - 4 hours
Chomper - Triggered by Large River Caimans and River Caimans
Chugga Bam Bam - Triggered by Orc Renegades
Cirrus Proudblood - Triggered by Storm Giant Elites
Construct of Ilmyrr - Triggered by quest
Crypt Protector Hobensnorl - 10 hours
Cumulus Proudblood - Triggered by Storm Giant Elites
Dark Dragon of Roachard - Triggered by Graal Roachard
Darkwing the Mindless - 10 hours
Deathcap - 20 hours
Der Schlaechter - 10 hours
Dunkelklinge - 10 hours
Dwarf Mine Master - 10 hours
Ewwww - Triggered by Rock Trolls
Faldon the Dwarf King - Does not spawn?
Frump the Cook - 1 hour
General Thrakk - 10 hours
Ghost of Lotor I - 20 hours
Ghoul Lord Yesoth - 10 hours
Goba Longblade - 1 hour
Gorgar the Pit Fiend - 1 hour
Graal Roachard - 4-7 days(36 of each scale to spawn)
Guardian Knight Lancel - 10 hours
High Priest of Kuthos - 4 hours
High Priest Volsey - 10 hours
Hook Hands - 10 hours
Hugla the Cryptkeeper - ?
Hungry Pumpkin - 1 hour
Icebane Coldstone - 10 hours
Ignis Fatus - 10 hours
Jormungandr - Triggered by 20 Green Scales and 1 Pristine Green Scale
Kezemde Maat - Triggered by 20 Gold Scales and 1 Pristine Gold Scale
Kind Kobold - 1 hour 40 minutes
King Mack Mack - 8 hours 55 minutes
Kobold Chief - Triggered by Kobolds
Krank Brut - 10 hours
Kritan the Accursed - 4 hours
Lava Mold - Triggered by elementals around spawn
Lord Cerebus - 10 hours
Lord Pallus Indoel - 10 hours
Lord Tserith - Triggered by 6 Evil Galls
Master Blaster - 1 hour 40 minutes
Medukala Stonegazer - 8 hours
Meklor the Immovable - 1 hour
Mingor the Merciless - 1 hour
Mukarramma - 10 hours
Mukarramma2 - Triggered by Mukarramma
Nithhogr - Triggered by 20 Red Scales and 1 Pristine Red Scale
Northern Frost Giant Cook - 4 hours
Northern Frost Giant Jester - 10 hours
Northern Frost Giant Librarian - 4 hours
Oak Rot - Triggered by mobs around spawn
Prime Creation - Triggered by Sand Golems west of Gatekeeper
Projection of Kuthos - 8 hours 55 minutes
Quetzalcoatl - Triggered by 20 Blue Scales and 1 Pristine Blue Scale
Ragaxm the Inconceivable - 1 hour
Raqlun the Strategist - ?
Ratnack - 5 hours
Ryonkah - 4 hours
Sand Sting - Triggered by Blood Scorpions around spawn
Sandy Claws - 10 hours
Schlag the Scorpiones - 1 hour
SlobberKnocker - Triggered by mobs around spawn
Snagglefang - Triggered by mobs around spawn
So Kang Kang - 10 hours
Son of Scalo - 2 hours 15 minutes
Spectral Dragon of Roachard - Triggered by Dark Dragon of Roachard
Stratus Proudblood - Triggered by Storm Giant Elites
The Great Pumpkin - 1 hour
The Great White Shark - Instant respawn
The Lich Prince - 4 hours
The Spore Summoner - 28 hours
Town Guardian - 1 hour
Umbergon - Triggered by Termigon Queen
Unkea the Wicked - 4 hours
Warlord Vinteruul - Tiggered by Statue of Warlord Vinteruul(every 3-5 hours)
Wasp Queen - Triggered by Wasps
White Fang - ?
Wicked Witch - 2 hours
This was put together from different fan sites, AE forums, and memory.
