The term 'monster' encompasses a large array of supernatural and otherworldly threats to humanity. Some of them are known to fall into subgroups listed below. In addition to that though there are many tales of unique horrors from the deepest forests and oceans - twisted beasts, malevolent spirits, elemental forces given form and those afflicted by terrible curses.
Demons
Ancient creatures that lurk at the threshold of our world seeking re-entry to resume their tyrannical reign over humanity once more. They are extremely individualistic but have been known to band together for a greater though still self-serving purpose. Though they have a strong affinity with fire, temptation and bargains are their greatest weapon. In their deals they may offer power and riches, but buyer beware as demons are always looking to bind humans to their service. Death does not come easy to them but they can be temporarily dissipated, banished from this realm or imprisoned. High ranking demons typically have larger horns and more intense colour to their complexion over their lesser peers, some of which could even pass as a Nafreet to the untrained eye.
Dragons
Also known as 'wyrm' these massive flying lizards are monsters of myth. A common feature in fairytales and old drawings, most 'evidence' of these creatures ever existing has been proven to be forgeries or simple misidentification of the bones of completely mundane creatures such as whales or elephants. Regardless, these creatures of legend and their alleged magical might still capture the imagination of the human subconscious. Drunken sailors will tell you how a draconic shaped shadow flew overhead right before a storm struck their ship. Outside of these wild tales of intense cloud cover, wyrm are believed to be extinct if they ever existed at all.
Fey
Humanity's greatest enemy, the fey are a group of long lived ethereal entities from another world connected to our own, known as the Feyrealm. They are organised into four seasonal courts and have a unified malice towards humanity and our works. Though some fey may come across as whimsical at times, many are terrifying to behold. They are always seeking to open up gateways between our worlds to allow more of their invading knights through. Fey have a strong affinity for magic and the natural world giving them command over the elements and able to deploy powerful curses upon their foes. They detest human innovation, as even their magical defenses break upon being struck by modern firepower. Despite all of this they are the most civilised of the monsters as they follow traditions, hospitality and even engage in politics. The fey are varied in power and looks. While the most wily are able to change their appearance at will, a fey's true form does look similar to humans. Often the colourful lines and markings of their court or their failure to emulate human customs and dress is what gives them away.
Giants
Scholarship widely believes giants used to roam Valyr but are now extinct. Discoveries of massive bones and colossal mountain strongholds survive as evidence of their might. These larger-than-life people likely died out a long time ago, though if the Umlaui myths are to be believed the will of the giant overlords may still live on through the dark magics they wielded. It is said that they were so tainted that any rock or soil that tasted the blood of giants became blighted and should be avoided at all costs.
Undead
Most typically associated with the reanimated husks of the Pale Scourge from The Deadlands, instances of restless dead and ghostly hauntings have been recorded at various sites across Valyr. Most undead are considered to be mindless unholy abominations animated by dark magic, or perturbed ancestors awoken by desecration or cursed with unfinished business. It is unclear whether it is truly possible for a mind to persist beyond death or simply some other power that spurs them to purpose. Few undead show a level of tactics and higher thinking that indicates intelligence or memory of their former lives. Undead vary in power depending on who they were in life and amount of dark magics coursing through them. If you run into someone that looks too pale it is best to shoot first and ask questions later.