As mentioned already on the main page, nearly every boss in the game is modeled after a sea creature.
Some boss names: Electric Fan, Fatty Glutton, Great Thing, My Home Daddy.
There was going to be a Darius III, but Taito thought that the original plot (Which involved the main characters returning to Darius, only to find it bombed to hell and back) was too depressing, and retooled it into a separate game called Metal Black.
Divorced Installment: The obscure Taito shmup Metal Black was originally devised as Darius 3, but it was deemed too depressing for the series' setting and so was spun off into its own game. Some artifacts of its lineage remain, most prominently in the inexplicable cameo of Yamato in the first stage and a few fish-like enemies.
Word of Dante: The gag boss in Akkanvader known as My Home Daddy's Girl got her name on a fan site.