Plenty of monsters for Spelljammer can be found in Monstrous Compendiums 7 & 9. However a DM sometimes wants to throw something new at the players. Changing or adapting old monsters to new forms is an great way to do this. The following are creatures to consider from the Monstrous Manual and the various Monstrous Compendiums volumes. This list is tailored to an Astromundi Cluster campaign, but can easily be altered for a more generic Spelljammer game.
Basilisk-Lesser (local species that can plane shift to the astral once per week and reverse its petrification gaze on its victims, this allows it to store up food for lean times), Bird-Wari (small asteroid dwellers, Rock Hoppers raise them as poultry), Cave Fisher (Asteroid Fisher, it can used its line to travel from one asteroid to another in a belt), Centipede-Giant, Dragon (of all the non-spelljammer dragons, only the Steel and Deep reside in the cluster. And only then in human/humanoid form in large cities), Gloomwing (both forms, they seem to be able to draw substance from the darker regions of the cluster), Grell (rogues are found from time to time), Hatori (these live in the larger asteroids that are composed mostly of loose sand or gravel), Hell Hounds (a prime material breed that the Antilans have domesticated), Insect (Asteroid Cricket, Fyrefly, Pernicon, Praying Mantis), Intellect Devourer (often used as pets/guard beasts by the illithids), Kirre (sometimes trained by the Antilans to attack illithids), Lizard (Giant and Fire, both can go dormant if air supply gets too low), Mammal (Chattur, Cooshee, Goat, Osquip, Rothe, Sleek, Stench Kow, Tyrg), Mist-Vampiric, Mole Men, Myconid, Owlbear (they can go dormant is air supply gets low), Puddings (several harmless varieties exist and are farmed by the native races, causing confusion to travelers, a successful agriculture check can tell the difference), Rust Monster (hunted by the dwarves, but propitiated by the elves), Snake-Winged (found only in the warmer inner sphere), Toad-Giant (local species that can dimension door once per week and go dormant if the air or water supply gets too low), Urchin-Land, Yeti (animal to low intelligence, hunted by the Thoric for its pelt). Note: undead are not listed since they are not part of the ecology. That does NOT mean they do not exist.
Abyss Ant (conjured by the Antilans much to their regret), Baelnorn (may guard elven interests), Cat-Winged (Greater and Lesser, frequently domesticated by the humans or elves), Elemental-Earth Weird (found on many asteroids), Elemental-Vermin (only as familiars of Elementalists wizards), Gnasher (both varieties are domesticated and used by the Scro and Pirates of Gith), Golem (Brain and Hammer), Jarbo (like the cave cricket, it has adapted to low atmosphere), Lock Lurker (with the proliferation of trade in the cluster, these pests have no trouble with survival), Lycanthrope-Werebadger (dwarves are susceptible), Magebane, Parasites (all), Stonesnake, Spider-Brain (introduced by the illithids to harass other races), Ulitharid (not a animal, but a noble illithid), Wolf-Vampiric, -Zombie (actually gnashers that the Scro have used special ritual magic on, winged gnashers are turned to vampiric and non-winged into zombie).
Addazahr (some Aperusa tribes know a nonmagical remedy to their bite), Bat-Spore, Beetle-Dragon, Chronolily (cultivated by Antilan mages who have journeyed to the Ethereal plane), Dinosaurs (found on some lizard-man worlds, mostly small species), Snake-Serpent Vine (imported by the elves whose priests have access to the plant sphere and thus are more capable of controlling them), Vizer's Turban (the native species usually takes the form of an Antilan mask), Wall Walker (adapted to low atmosphere, but very rare), Webbird.
Ash Crawler (found in the asteroids nearest Firefall), Bird-Piranha (lesser and greater), Blackball (although very rare in the extreme, they seem to be spawned when a wrinkle collapses), Choker (a mutated from of goblinoids that the Scro sometimes used as shock troops), Crone of Chaos (the only native hag species), Frost Salamander (found in the ice belts of the Fringe), Ghostly Horde (the guardians of a great Thoric warlords burial cairn), Golem-Rock (created by a special 5th level Earth Elemental spell granted to priests Chodak), Lizard-Rockhome (imported by the dwarves as a beast of burden), Manikin (sometimes created and used by the elves), White Fang (native to the Fringe, hunted by Thoric fur traders),
Bastellus (seems to prey upon all races equally), Bussengeist (some spirits from the great cataclysm still wander the cluster), Doppleganger Plant (seems to trouble all but the elves, who are immune to their mental powers), Lycanthrope-Wereraven (some seem to have migrated here from Grayspace, their original home), Vampire-Dwarf, -Elf (whenever one of these races become stranded on the Isle of Banshees), Vampyre (can be found among the Antilans and Varan and rearly an Aperusa tribe), Vampire-Illithid (also a victim of the Isle).
Akikage (if the DM has an oriental colony), Animator (all three), Beetle-Scarab, Figurine (all types, often created by Antilan Dweomercrafters), Flees of Madness, Golem-Snow (a similar version can be created by some Thoric wise women), Leech (both magical and psionic), Lich-Psionic, Plant-Bloodrose (cultivated by some Antilan nobles as an exotic luxury, the fragrence is magnificent), Shadow Asp (frequently conjured via a special spell of the Seltain's priests), Virus (all types can be found, but combustion and crystal are more often found in the Antilan empire, psionic and shadow are more often found in the Illithid holdings)
Minion of Set (Minion of Seltain).
