Looks like there's a group of unused Imp battles 131-136. 184 - Unused Imp. 204 - Unused Imp. Same with 208. Same with 216-220. 295-296, the same.
Unmistakably Underworld/Lunar formations begin at 257 or in Hex 0100.
TrapDoor's are interesting... in their battle formations their transformation is listed as plainly as if they are there. Which means that it may be something special in the formation (maybe the Mystery Byte being set to 20?) or something else perhaps?
Seems that Moon Formations begin at 369.
377-383 are filled with Dummy's.
405-406 are both the Tricker, nothing different between the two formations.
417 is just an Imp.
421-423 just Imps.
430 - Odin fought in the Underworld? Interesting...
425 and 439 are both Golbez/Shadow. One or the other must be an unused copy.
441-443 - Imp/Dummy's.
457-472 - All filled with Imp.
502-506 - All Imp.
512 - Imp.
Well... that's a lot of unused/dummied formation space.
In total...
131-136 - 5
184 - 6
204 - 7
208 - 8
216-220 - 13
295-296 - 15
377-383 - 22
417 - 23
421-423 - 26
441-443 - 29
457-472 - 45
502-506 - 50
512 - 51
Good work Square! 51 of your 512 - 10% of your chosen space is unused (or was taken out since some were no doubt filled with the programmers)
Well I guess that's more space for us to fool around with, haha. Looking forward to seeing the Formation Chooser for Maps!

It seems that Mystery Flag 2 must deal something with displaying multiple monsters. When Mystery Flag 2 is set as Off the first monster type will show up fine, but any additional ones will be a glitchy mess.

2 Mystery Flag 3 is used Exclusively by the Zeromus Formation. When activated the normal monster graphics will glitch when you enter battle, even though Mystery Flag 2 is still activated.

3 Mystery Amount is interesting. When it is not set and you have a Monster Spawning enemy, the spawned enemy will be present automatically. When this value is set to 1 the spawned enemy will not be present until summoned and notably nearly every encounter that has a summoned enemy has that value set to 1.

4 Mystery Amount, when set to 2 will hide a group of 2, but from what I could tell, setting it to 3 did not hide a group of 3.

5 What do we know about the Auto-Battle functionality? Outside of its proper environments it seems to just cause the player to use the first spell on the list, which is blank. It doesn't seem to do anything with Arrangement or the Mystery Byte, maybe it's formation number itself? Well I guess we'll find that one out soon enough.