Aboleth are arguably the strongest candidates of the 'odd' monsters for such treatment. I could see multiple different versions, maybe putting the veiled master in as their 'affiliated monster'
But for instance, what sort of treant statblocks would you expect to see? Would you think the treant section to fill up the same page count as fire giants or drow?
Basically, I wasn't saying "you shouldn't have X in the Monster Codex II", I was asking how you would implement it, because it doesn't seem as intuitive as say, hobgoblins and orcs.
Personally, I'd say that the treants would be organized in fewer numbers than the tribal humanoids. I imagine they would be organized in something akin to a druid's grove, with 4-12 normal/low-character level treants and 1-3 elder treants of various spellcasting levels (or just advanced HD with the various class simple templates from MC1), especially in druid, shaman, or sorcerer. Dryads and other similar fey could also be included in the grove, as might plant creatures similar/aligned with the treant.