Overall, the opponents that Fafhrd and the Gray Mouser face are sorcerers, mad priests and priestesses, cults, other rogues, the Thieves’ Guild, powerful personages, and even Death himself; but the twain also confront many monsters in their adventures.
There are many animals that become monstrous either due to sorcerous summonings, occult, or supernatural causes. First and the most infamous are the Rats. We are not aware of the collective intellect of the Rats when we first meet them in Ill Met in Lankhmar. There is the rat familiar of the wizard Hristomilo, Slivikin, who along with his rat-kin help kill the first-loves of Fafhrd and the Gray Mouser, Vlana and Ivrian. At this point in time, they are ‘merely’ sinister. Only when we meet up with them again in The Swords of Lankhmar do we see the true danger they pose against Lankhmar and could pose again someday.
There are also the Hounds who haunt the old man in The Howling Tower and fight Fafhrd in the astral plane. Then there are the Birds of Tyaa of the Claws from the Night who terrorize Lankhmar for three months before Fafhrd and the Gray Mouser force the cult-priestess, Atya, and the birds of Tyaa to flee to the Mountains of Hunger. In The Mer She, a Behemoth named Deep Rusher nearly sinks the Seahawk.
Among the more traditional monsters, we have the Beasts from The Bleak Shore that the twain are unwillingly pitted against.
Shambling things, erect like men but taller, with reptilian heads boned and crested like helmets, feet clawed like a lizard’s, shoulders topped with bony spikes, forelimbs each terminating in a single yard-long claw.
Sea creatures that look like over-sized black cloaks overcome Lavas Laerk and his crew while Fafhrd escapes and the Gray Mouser watches in The Sunken Land. An Iron Statue of the Devourers challenges Fafhrd in one of his most taxing duels ever in the Bazaar of the Bizarre. From When the Sea-King's Away we witness a squid wielding a bastard sword in each tentacle menace Fafhrd, while the Gray Mouser is ambushed by three dead sailors. Lastly, there is the great Snow Serpent that guards the way against Obelisk Polaris to Stardock in the story Stardock.
The undead make for the most horrifying monsters witnessed in Nehwon. Not even the twain dare to directly assault the walking mummified skeletons of either the Dead Master Thieves or the Gods of Lankhmar. Those that did invariably died, except for one noteworthy exception, the Rat Plague from the Swords of Lankhmar. Their sheer numbers and the use of fire were too much for the Gods of Lankhmar. The only undead that were not a match for Fafhrd or the Gray Mouser were the three dead sailors from When the Sea-King's Away.
One might be tempted to include the Ghouls from The Swords of Lankhmar among the monsters. However, one might see the Ghouls as more of a different race of humanoids rather than your traditional monsters. Despite their outward appearance of invisible flesh and visible bones, they are very human. I think the same goes for the inhabitants of Simorgya, the Invisibles of Stardock, and the tribes of Ice Gnomes. It is very tempting to say the same of even the Rats of Lankhmar Below.
Some monsters are only mentioned in passing in the stories: