Dropper enemy in Figaro Castle?

[Lux]

Is this a missable rage if you do not encounter one here? I cannot believe I didn't encounter them.

[Lux]

Oh, holy crap, you can re-enter this place. I swear, the completely out of shape guy that grunts every time he hits a lever stopped you from re-entering this place. I think i'm still on track for a 100% bestiary, woot!

[Atari]

Don't forget that (spoilers) you can fight some missed enemies in the soul shrine later.

[LadGaga]

Most of the WoR is permanently open to you.

With the odd exception.

Cyan's dream being an obvious one.

[Gnord]

Additional WoR Missables:
Crumbling House, where you find Sabin(WoR). The "Zokka/HermitCrab" enemy there can easily be missed, along with Scorpion and Nightwalker, but the latter two are common, so it's quite hard to miss those. Zokka is NOT available in Soul Shrine.

Additionally, the "Rafflesia" enemy in Owzer's Mansion is missable, and be forced to find it in Soul Shrine.

Finding Dropper:
I have seen 3x Dropper in the engine room. I haven't seen it in any other rooms, despite dozens of battles.
I have also experienced long runs where I could not find Dropper, or other rare encounters, where they're known to be present. I found that for rare spawns in general, saving to the main-screen, re-loading, and going quickly to a location certain where they spawn often helps. If you're uncertain, reload your save for each room, and try multiple battles in a single room before reloading and moving to the next. (when possible)