Bossmonster
Frightener:
A Frightener is the very first boss monster Geralt has to fight against if he decides to do so. A Frightener is a mean beast summoned by a mage, in this case it was Savolla who summoned it. It is looking like an insect, if a bit bigger and it is vulnerable by making loud noise in a high frequency and then attack it. It is sensitive to steel and silver, skinnable alchemical ingredients are Frightener's Eye and Frightener's Claw.
Hellhound:
In the Outskirts of Vizima Geralt encounters a Hellhound, also known as "The Beast". It is something like a demon from underground and it appears where dishonorable people live. It can summon Barghests and it traps its victims in a circle of fire. It is sensitive to steel, skinnable alchemical ingredient is Trace of the Beyond.
Golem:
A Golem is made from stone and animated by mages. In the swamp Geralt will meet this monsters, they can be easily defeated by a lightning bolt from three nearby stone pillars. However, normally they are mostly immune to steel or silver and to the most magical effects, excluding the witcher's Igni spell. Skinnable alchemical ingredient is a Golem's Obsidian Heart.
Werewolf:
A Werewolf is a cursed human who is turning into a hairy beast during the full moon, they are also known as Lycanthropes. When Geralt finds a Werewolf in the game, he has to decide to kill it or to remove the curse (I'm not sure, if removing was possible here). Werewolves are only sensitive to silver, skinnable alchemical ingredient is the Werewolf Fur.
Kikimore Queen:
A Kikimore Queen is looking like an insect and they are creating colonys like ants do. It has a very hard exoskeleton and is resistant to all effects. It is said that if a witcher encounters a Kikimore Queen, the only way to fight her is to run away. It is resistant to steel and silver, skinnable alchemical ingredient is the Kikimore Queen's Nerve.
Dagon:
The Dagon is an underwater god and a mythical creature. Some people of the fishfolk called Vodyanoi worship this mythical god. It inhabits the Lake Vizima near the village Murky Waters and the Vodyanoi had build a temple for it on Black Tern Island. Dagon is a god and for this completely invulnerable, however, he can be killed by defeating his worshippers. Skinnable alchemical ingredient is the Dagon Secretion.
Striga:
The Striga is always a woman (princess Adda in this case) on which an evil curse is laying. A Striga is a monster full of hate against all living beings, it has claws and is very agile, when the curse can be lifted the women have to wear amulets to prevend them from turning into a Striga again. In the game Geralt has to fight this Striga and kill her. It is vulnerable to silver, skinnable alchemical ingredient is the Striga Heart.
Koshchey:
A Koshchey is also a summoned monster as the Frightener is. Born from strong magic of an experienced mage it is very hard to kill. Geralt has to fight such a beast, that looks like a mix between a crab and a spider with a hard exoskeleton, in a chamber where he can only fight or die, there's no way out until the beast is dead. A Koshchey is sensitive to silver, skinnable alchemical ingredient is the Koshchey Heart.
Zeugl:
A Zeugl is a beast that is growing up between the dirt and garbage in large human citys. It has tentacles and looks like a big octopus if a bit more aggressive. Geralt encounters such a beast in the sewers of Vizima and he knows that this monster is invulnerable by steel or silver and can only be killed by cutting off its tentacles. Skinnable alchemical ingredient is Zeugl Venom.
King of the Wild Hunt:
The King of the Wild Hunt is the most spectacular specter in this game. Well... he is at least something like a specter, a rare ghost creaping up from the shadows into Geralt's life and with every encounter Geralt learns that he is more his doom than a monster. The King of the Wild Hunt is the leader of a horde of wraiths raiders. He is Geralt's destiny even if Geralt declares that he doesn't believe in destiny. The King of the Wild Hunt is the last big boss monster Geralt has to fight against. Not skinnable, no alchemical ingredients.
Other Monsters
Drowner & Drowned Dead:
Drowners are green and slimy humanoid monsters, they are humans who are drowned in water or during storms. They drag people down into mires or bubbling eddies and they are easy to kill with normal weapons. A Drowned Dead is a dangerous variety of a Drowner. Skinnable alchemical ingredients are Drowner Brain Tissue, Ginatz Acid, Cadaverine and Drowned Dead Tongue.
Ghoul & Alghoul:
Ghouls are found in cemeteries, crypts or on battlefields where a large amount of rotten flesh exists. They hate bright light and hunt at night, if they are hungry they don't hesitate to feed on living beings. An Alghoul is the more dangerous variety of a Ghoul. Skinnable alchemical ingredients are Abomination Lymph, White Vinegar, Ghoul Blood, Cadaverine and Alghoul Marrow.
Echinops & Archespore:
Echinopsae are carnivorous plants and grow underground. If they recognize a victim near their place they appear and attack. Archespores grow in places where a crime was committed or on tombs where victims of a crime were burried. Skinnable alchemical ingredients are Echinops Rootstock, Spores and Archespore Juice.
Bloedzuiger:
A Bloedzuiger is a big leech, it spills acid and is immune to poison. Bloedzuigers are living in swamps and when killed that beast explodes in a cloud of acid. Experienced witchers can avoid this acid cloud. Bloedzuigers are sensitive to silver and a witcher's Igni sign. Skinnable alchemical ingredients are Abomination Lymph, Albar's Crystals and Bloedzuiger Blood.
Giant Centipede:
Giant Centipedes are like big earthworms living underground, they appear and attack if a victim comes near their path. They are found in forests or meadows and their body is littered with spikes. They are blind but seeing all with their nerves, in combat they knock their opponents down and poison them. Skinnable alchemical ingredients are Tracheae, Venom Glands, Toxin and sometimes a Chitinous Carapace.
Wraith:
Wraiths are ghosts that are found near their graves or their old homes. They are restless and hunt everything that lives. Sometimes they are summoned by magic and witchers have to put them to rest. Skinnable alchemical ingredients are Death Dust and Ectoplasm.
Wyvern & Royal Wyvern:
Wyverns are great flying beasts that are commonly mistaken with dragons. When they swoop down from the sky hunting their prey they don't make a difference between a sheep or a man. Royal Wyverns are more aggressive and venomous as normal Wyverns. Skinnable alchemical ingredients are Toxin, Wing Membrane and Venom Glands. Sometimes Wyvern Meat and Wyvern Eggs can be found on their bodies.
Ifrit:
An Ifrit is a variety of Genies that can be summoned from the Plane of Fire by mages who have achieved mastery in controlling the element of fire. It does not grant wishes like normal Genies but the summoning mage can force them into servitude and let them kill whole parties. Skinnable alchemical ingredients are Pyrite and Ectoplasm.
Cockatrice & Basilisk:
Cockatrices are ornithosaurs that live in caves and dark cellars. Their gaze turns the living into stone and it is said that an adventurer without a mirror is lost, however, witchers reply that it is far better to smash the mirror at the creatures head. Basilisks are the greater variety of Cockatrices and even more dangerous, their pure breath is venomous. Skinnable alchemical ingredients are Toxin, Cockatrice Eye, Cockatrice Feather, Venom Glands and sometimes a Basilisk Hide.
Greater Brothers & Greater Mutant:
Greater Brothers and Greater Mutants are mutants created through experiments with the witcher's mutagenic potions. While Greater Brothers are super-humans and warriors serving the Order of the Flaming Rose, the Greater Mutants are failed experiments created by the Salamandra. Both are slow but strong and they are sensitive to steel. Skinnable alchemical ingredients are Pituitary Gland, Beast Liver, Beast Fangs and Mutagen.
Skullhead:
Skullheads died out centuries ago during the last ice age. However, Geralt encounters one in the Ice Plains, where he has to fight against his last enemy. Skullheads were aggressive barbarians with a low gravity center which makes them hard to knock down. Skinnable alchemical ingredients are Beast Liver, Beast Fangs, Skullhead Talons and Tendons.