The bosses, they all seem to be morphed in some way. Diababa, Deku Baba. Firus, patriarch of the Gorons. Yeta, big snow monster. Little Gohma, seen at the end of the battle, big Gohma. Aragarok, well it seems like a Karagarok. But what about Morpheel?
Well I think he was a normal eel at first, then BAM. Giant mutant eel. Or loach.
Of course, the mutation made him have weird ectoplasmic tentacles and a giant mouth, but... eh.
So what do other people think?
Morpheel was like a giant piranah form earlier in the temple, and Stallord was just a big skeloton of a dragon. Fyrus was also a rip-off of the Balrog, almost.
Giant pirrahna? You mean the skeleton fish?
*SPOILER WARNING*
Still, you got to remember that the power of the Fused Shadows is what makes those monsters. It's only natural that they're warped because of that fact.
*END OF SPOILERS*
It only takes into consideration the first three bosses, the rest don't count:
Stallord - revived by Zant
5th dungeon boss - Magic Mirror cursed Yeta
The other two are unknown, but Zant probably made them and gave them the magic mirror shards.
Midna actually says that Gohma was that way (she says ugly) because of the Mirror Shards.