The floater is a large, soft-bodied invertebrate, named after the flotation sacs (frequently abbreviated to just float sac) that, contrary to its name, allows it to hover in the air. Noxious, flammable gases are generated by the creature's tissues, then stored in these large bladders, giving it its distinctive appearance. The sacs surround a mouth, which contains a tentacle terminating in a primitive, cross-shaped maw. When the floater attacks its prey, the maw is launched forwards, spiraling through the air and attempting to bite down on the target. Despite its ungainly appearance, the floater is fast, using its flexible tail to slither along the ground. Finally, they are oviparous, equipped with an ovipositor to lay eggs. Fortunately, floaters lack any meaningful intelligence: Their rudimentary ganglia are enough for them to recognize prey and members of its pack, but they are unable to work together as a pack, simply swarming whatever target they come across. However, that simplicity also makes them tenacious and difficult to put down. Their one disadvantage is that the flotation gases are flammable, making energy and flame weapons highly effective against these combustible creatures.
Floaters are durable creatures, and move very quickly due to their unique ability to hover. They often travel in groups with super mutants and centaurs, and can pose a great threat to anyone unprepared for combat.
If normal floaters are bad, then the nasty floaters are far worse. Even more durable and tenacious than their counterparts, they are indeed capable foes. Just like common floaters, they are very susceptible to fire, due to their flammable gasses.
“I think Floaters are cool critters in the Fallout universe, but, like Centaurs, they have really weird bodies that require unique animation rigs. From an animation perspective, limbless flying things tend to be really cheap (usually), human-like bipeds are cheap, quadrupeds aren’t too bad. Once you start getting into creatures with multiple heads, snake bodies, tentacles, etc., they get really expensive. IIRC, we talked about having Floaters but we just didn’t have the animation time for them.”— Joshua Sawyer on the Frog Helms Fan Club
Floaters are the creation of Jason D. Anderson. Notably, the creature was known as the "Sphincter Monster" during development after Anderson had used a pornographic image for its mouth.
↑Fallout 2 Official Strategies & Secrets p.26-27: "Floaters hover by, manufacturing and storing noxious gases in their flotation bladders. They're stupid, but move fast and are very difficult to kill. Floaters will try to surround you, so keep your distance. The gases that allow them to float turn them into a wonderful flambé when you crisp them with fire or energy weapons."