Hmmm... ok, I will only post reasons on a few of them... ok?
Twin Tania
Golem-Hey, I like him! Help you in battle... and you have to help him as well. Is he really a boss? Ah, well... whatever.
Grand mummy- Didn't he drop that cool curse ring? Anyway... he's a mummy!
Alte Roit- Much stronger.
Carbuncle- For once this summon doesn't look cute. But it's still usefull at times + harder battle in my opinion.
Azulmagia- Cool blue mage boss.
Halycanos-Seriously, he's the FROG PRINCE!!!!
Siren
Tyrasaurus- Undead Dinosaur!!!!!
Catastrophe
Minitaurus
JuraAvis
Garula-He's a really nice baby, so please... don't be mean to him.
Archeovis
Enkidoh- Gilgameshs partner.
Mellusion
Iron claw
Ifrit
Odin
Byurobolos-This boss can be pretty hard sometimes.
Gilgame- ALL THE WAY!!!!! GO, GO, GO!!!!
X-deth-Soul- Reminds me of Golbez from FFIV
Adamantaim
Dragon grass
Kimabrain
Red dragon
Invisiable
Pantera
Levithan- Cool... but it's sad that you never could face that monster who was swallowed.
Earth Crystal
Atomos- The swallowing thing was fun.
Omniscient