Before I go any further I should mention that what follows is all based on the original 2ed AD&D version of spelljammer. All of the political stuff may well have changed with the 3e version. I am using the 2e stuff because I have all the source material from then and if what wotc did to the monsters is anything to go by...
In spelljammer monsters and creatures can be divided into 4 distinct types. Those found only in the phlogiston, those only found in wildspace, those found only on specific worlds and those that cross boundaries. One thing to note is that quite a large proportion of monsters found in space are vast. Some are the size if moons others are even larger. While these work fine in PnP campaigns they are obviously too large for NwN.
Of those found in the phlogiston, a lot are simply normal monsters that the material of the flow has somehow twisted. There are however some that are totally unique to the phlogiston. Of these, Stellar Dragons are probably the largest at a maximum total length of 3,000,000 feet long (I am not inclined to go to the bother of converting this to miles/kilometres/whatever).
Before continuing I need to mention a little fact about worlds in wildspace. In spelljammer worlds fall into four different classes. These are Fire worlds, Earth worlds, Air worlds and Water worlds. This goes a long way to explaining the diversity of life found on the worlds of wildspace. The star at the heart of a planetary system is obviously a fire world and usually has a direct connection to the plane of fire. Contrary to what you might expect most of them have creatures living on their surface. Water worlds have mostly aquatic life forms while air worlds have mostly floating (aka living balloons) and avian life. While there is no directly comparable water worlds in our universe, the gas giants are a (very) rough equivalent to air worlds. This leaves earth type worlds. These are what we have come to expect being equivalent to our world. As such they have a greater diversity of life than all the other world types combined.
The creatures that can be found (or in some cases infest) in the depths of wildspace itself vary in size from those that are the size of barnacles and ship-worms up to minor planet size. Mostly they are the wildspace equivalent of sea creatures here on earth.
Finally we come to those creatures and races that can be found in two or more of the locations previously mentioned. In the main these are either intelligent or they are transported by intelligent creatures. The intelligent creatures can be further sub-divided into two further groups - those that can be found in a normal terrestrial setting and those only found in the wider spelljammer setting.
With a couple of exceptions (detailed shortly), if you look hard enough you will find any intelligent creature (and their livestock) somewhere in the spelljammer setting, even if they are only to be found enslaved somewhere. One thing to note is that not all individuals of these races behave as you would expect. One exception immediately springs to mind. Large Luigi is a bar owner on the Rock of Bral (an asteroid city-state). Luigi does not get a lot trouble in his bar mainly because he is a Beholder. The first of the exceptions are any creature unique to the Dark Sun setting. This is because the rules of magic within that setting prevent any portals from opening in the crystal sphere associated with Athas. Because of this, creatures from there cannot be found in the wider spelljammer setting.
The second exception is more specific and relates to a single world - Krynn. This is down to a decree by the gods of that world. While it is possible to land there, getting back into wildspace again is nearly impossible. The only god to flout this rule is Reorx who encourages his Gnomoi and Minoi (Krynnish gnomes) to do so. There is one further exception here in that certain kender have been known to stow-away on board gnomish ships while under the influence of wanderlust. Mention should be made here of the ships created by these gnomes. Just like the planet bound brethren's creations, each one is unique. They employ weird and wonderful mechanisms for propulsion, many of which require gigantic rubber bands that need to be periodically wound up. In order to save the gnomish crew from having to do this they bred the Giant Space Hamster. When they had done this they discovered that they found the meat of these beasts to be delicious and named it Spaham (down Vikings (Monty Python reference)). Unfortunately for the rest of the spelljamming universe their breeding program didn't stop there...
Some results of the breeding programs.
TR