I never looked at this before, but based on how the data is structured, it looks like Dream Robe is definitely a possible drop after the first time beating the Ch 5 Twin Snake, and Dragon Scale and Flame Casque are 1-time drops.
mt. path, twin snake (2nd) block 12 Chance 20
mt. path, twin snake (2nd) b12 Treasure 1 Custom Robe
mt. path, twin snake (2nd) b12 Treasure 2 Silver White Robe
mt. path, twin snake (2nd) b12 Treasure 3 Dream Robe
mt. path, twin snake (2nd) b12 Treasure 4 Blessed Robe
mt. path, twin snake (2nd) b12 Treasure 5 Fine Robe
mt. path, twin snake (2nd) b12 Treasure 6 Old Silver Robe
The first item is definitely the item given out the first time you beat that boss. Treasure 2 through 6 are the random drops later on, and the Chance is related to the chance an item from this bucket appears. 20% chance? But Dream Robe is 1/5 and we can't tell from this list if there are different chances for the items listed in each bucket are selected.
Another item of interest, Blood Armor, is supposed to be a common drop by the Copper Sun. I definitely never farmed this guy due to the amount of walking needed.
cyndar, copper sun block 7 Chance 40
cyndar, copper sun b7 Treasure 1 Armor Of Rage
cyndar, copper sun b7 Treasure 2 Fine Armor
cyndar, copper sun b7 Treasure 3 Blood Armor
cyndar, copper sun b7 Treasure 4 Blessed Armor
cyndar, copper sun b7 Treasure 5 Premier Armor
cyndar, copper sun b7 Treasure 6 <ITEM>