Bosses

Before progressing into the next area you have to defeat the boss of the current area you are on. Each boss is met at a given kilometer, the amount of kilometers between each boss is between 4-10 km. You also have the possibility to gain the boss as a in-game pet as well as receive either a weapon, armor, shield, or an ornament, but only once at a time an at random an based on your luk.

Bosses will only respawn if you enter the Castle at any time (includes the player dying and returning back), or by using a Warp Point to warp.

A little fact, some of the higher bosses can move from their spot an slowly advance towards you if you initiate their name sequence an then walk away. If they come up to you and you're next to the previous area, the boss will push you into that said previous area. While after a certain level, low level bosses will only attack once after you initiate their name sequence. From what I have witnessed between 8km and 14km bosses, both only attacked once even when standing right beside them. (Might be attributed to your Skills too.)

Boss List
Starting from first to last. Total of 67.
 * Bite Dog
 * Goblin King
 * Ostrich
 * Beast King
 * Chameleon
 * Man Eater
 * King Mush
 * Gorilla
 * Mystic Knight
 * Balloon Ghost
 * Nine Tailed Fox
 * Killer Ant
 * Slime King
 * Blade Horse
 * Killer Serpent
 * Stick Slime
 * Shoebill
 * Lizardman
 * Graggart
 * Naumann
 * Skull Knight
 * Venom Slime
 * Cactus Ball
 * Sand Warm
 * Anubis
 * Behemoth
 * Volganus
 * Kappa
 * Hippopotamus
 * Battle Crab
 * Soldier Bug
 * Blazzard
 * Dagon
 * Undine
 * Clione
 * Golem
 * Moler
 * Ahriman
 * Satan
 * Yeti
 * Guardian
 * Cyclops
 * Big Eye
 * Death
 * Gargoyle
 * Blue Knight
 * Cerberus
 * Tall Bat
 * Wyvern
 * Dragon
 * Dragon Rider
 * Plague
 * Machine No1
 * Machine No2
 * Ooze
 * Ashura
 * Tyrant
 * Tired Dragon
 * Ifrit
 * Despot
 * Werewolf
 * Death Knight
 * Machine No3
 * Gigantes
 * Lucifer
 * Demon Wall
 * Lucifer Ex

Bugs
In the current version of the game (as of June 1st, 2018) there is a bug where if the player dies on any screen, and before respawning back at the Castle, you can close and reopen MinuteSlash to respawn on the screen you died at. This method is highly advantageous if the current boss you are fighting is tough and you keep dying.