Two of the main kinds of monsters are found and sometimes fought by Twinsen in his adventures. One kind are blue monsters that only show their heads above the gas, and use their long necks to attack its enemies by biting them. The other kind look like walrus with little wings, and shoot fire balls through their mouth.
There is also an eel-like gas monster with four eyes, but it's only seen in cinematics or flying far away.
Twinsen finds several of the blue monsters scattered through the islands of the surface of Zeelich. In Otringal, several of the emerge near the spot where Baldino crashes his spacecraft. In The Island of the Celebration, there is one near the crystal rock. In the Island of the Francos, Twinsen finds a rather large blue gas monster that isn't hostile. And finally, several of them are seen in the passages of the Island CX.
He also finds several of the walrus-looking monsters. A couple of them emerge in the passages of the Island CX, and one of them appears in the arena of the Island of the Mosquibees, which is killed by Twinsen with the help of the Wannie's Glove, to get the key to the chest which holds the Fragment of the Mosquibees.
Dark Monk's prophecy announces that the gas monsters will disappear when Zeelich is restored to its previous form, so it can be assumed that they become extinct when the prophecy is fulfilled at the end of LBA2.