New Monster Ideas

[Bearic]

I still love these guys: http://en.wikipedia.org/wiki/Gas_spore
 
Giant bats, could be fun - like a ten times larger bat creature.
 
Some sort of Dire mole, for all the gnomes to fear.
 
Random Kuo-Tau hunting parties in some of the more difficult areas.
 
Maybe a goblinoid crossbreed, similar to an Oorg, but taking a goblin and hobgoblin together, or something.
 
I've yet to see any Derros around, which used to be in eariler versions of the underdark, to my memory.
 
Magmin are cool, like lavamen - could use the same barkskin like mudmen but with a red glow effect; give them fire damage, that sort of thing.
 
half-ilithid crossbreeds could be interesting, like the lizardfolk kind.
 
Dekanter goblins could be cool; think rhino goblins but with fire and cold resistance and light regeneration.
 
Some Grells would be sweet, if hard to show correctly.

[granny]

Skriaxit - Skriaxits (cloud monsters, perfect for Lowerdark) (also known as blackstorms and living sandstorms are malevolent air elementals. They are retributive spirits summoned from the Elemental Plane of Air millennia ago by ancient gods. Skriaxits whip up powerful sandstorms that can rip the flesh from their victims within minutes.

[scrappayeti]

I had an idea for an insect, leech or spider swarm.

This swarm would be from the nastier parts of the underdark, and its purpose would be to coax players to travel as a team rather than solo.  

The invertebrates would be in a swarm of five+, and be stealthed to a low-moderate degree. Their defining feature is that they do the spell 'heart clutch' on hit. This represents the creature latching on and paralysing the prey, then slowly sucking out the blood.

The beasties would be pretty weak, and a team could clear them off a paralysed player pretty quickly, but alone, they would be terrifying, as you roll fort save after fort save trying to get free, as they nibble you to death.

The could be leeches (small version of a snake), ticks (the foot based ones we already have) or mites (small spiders). To make them distinct, they might have the all black shadow shield look to them, to make them appropriately ominous and clearly different to the less dangerous little creatures that abound.