There are certain maps where 'special' monsters spawn if certain conditions are fulfilled, or only rarely, like example;

Fesp in Yuidora Woodland
Rocktoise in Training Ground
Labrador in Scorching Caves
Grollshell and Mitorunt in Lake of Souls
Treasure Putetto in Jungle Tribeland (getting the third item seems to take an awfully looong time even with Mul's Fill... Edit; Got it;Treasure Chest)

^not really much of a problem for someone who's thoroughly collecting everything. They're even all mentioned in the descriptions.
Reaper in various places is an exception though. (Do the Reaper(s) actually become beatable at some point in the game? (at ch. 4 currently))

...But then I meet Spina Protol (Blockaded Area, in Kania Wasteland). WTF? Unless I'd taken time to test if anything happens if I wait the turns out, and right, what do I see appear?
Just like the Reaper it's not listed in the description, so they may share same MO? Except Spina Protol is actually beatable (Had to stain one character's profile with a 'skull' for the first time to take it down though )

Any 'special' ones I've missed? (ch. 4 currently)
Any I should take a note of in further chapters so I don't miss them?

That map with the ton of Trolls (where you have to go to save Crayanne). Go back to it in freeplay. The portal at the very top will spawn a Zombie Goble after 15-20 turns.

Spina pops up in various places actually. He always appears on the last few turns though.

I think it'll pop up even more after you're done with the reaper(You'll be done with it soon enough, iirc it's necessary to get past chapter 4) as a replacement for the reaper.

Haven't really noted any of the others ^^; Though Rocktoise appears in several of the earlier Yuidore Mine maps. The second freemap that you get has it spawn in the lower left portal.

I'd argue Mifu Dragon is somewhat missable since you have to go out of your way to find it. It's in a hidden passage/room north of the starting gate in the Fire Shrine. Although any of the dragons are kinda annoying to farm for materials anyway(3 drops, only 1 spawns per map ~_~;)

These are not 'special' monsters - they are regular monsters(except Spina which is somewhat special) that appear later(mostly in the next chapter even) and can appear rarely in several maps as part of the clearing conditions.

I think that was the point the OP was trying to make, as they are monsters that require special conditions to appear for certain maps.

I may be wrong though.

What's the condition for the Mitorunt in Lake of Souls?