Where most people see danger in monstrous creatures, some see dollar signs. When a monster is the source of a substance (often a drug) that can be sold for massive profits, or bartered for power over others, then people looking to cash in on its byproducts may prove more dangerous than the creature itself. If the source-creature is non-sentient, expect it to be kept captive as a resource, until it escapes and goes on a rampage. If it's intelligent, expect it to bind hordes of Mooks to its service with its "gifts", to say nothing of people in positions of political influence. Either way, expect those who crave its byproducts to stand between it and any pesky do-gooder monster hunters who might cut off their supply.
Compare Milking the Monster, where it's the very presence of the monster that works to someone's financial or social/political advantage. Cases where the monster is self-aware and its "gifts" are plot-enabling are Sentient Phlebotinum. Sometimes overlaps with The Power of Blood.
Vampire blood has often been depicted as having the power to extend the natural lifespan of ordinary humans, allowing them to bribe mortals to their service with drops of blood.
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If they weren't a massively-profitable source of marketable food for humans, the B-Ms of Bio-Meat: Nectar would've surely been written off as a bad design and destroyed as soon as their appetite for people was noticed. Instead, the Corrupt Corporate Executive whose company created them writes off one catastrophic BM escape after another, and is perfectly content to slaughter innocents and his own son to protect his "product"'s public image.
In the French comic L'Imploseur, the miracle drink Ultra which boosts reflexes, muscles, etc. turns out to be the blood of a goat-human hybrid.
One of the reasons Gargamel goes after The Smurfs is because they are an ingredient in a formula for the Philosopher's Stone.
In the Alien Verse, aliens produce Royal Jelly which has the same role for this species as it has for real-life bees. However, it is also an extremely valuable substance in human society, used as a powerful and mind-enhancing drug for wealthy individuals. Since the only source of Royal Jelly is often deep inside an alien hive, collecting it can be very dangerous. The Hive mini-series details such an operation.
On Petes Dragon, Doctor Terminus wants to get his hands on Elliot (the titular dragon) to be made into medicines.
Dune's sandworms are the source of the invaluable Spice, without which galactic civilization would collapse. While keeping them captive is unfeasible to say the least, the need to preserve their species runs at odds with the Fremen drive to make Arrakis more liveable for humans, making it a major political issue.
In "Andy Warhol's Dracula", part of Kim Newman's Anno Dracula series, the central character is a drug dealer whose product uses vampire blood as its key ingredient.
In Alastair Reynolds' Terminal World, the world has been divided up into differing areas of technology, where the laws of reality are literally different from one are to the next, limiting how advanced certain items can be in certain areas. For the people of this world, traveling between these areas involves suffereing from the debilitating "Zone Sickness" that, in severe enough cases (or too fast of a transition between different zones) can kill. Fortunately, creatures called Carnivorgs can synthesize a medicine that alleviates the sickness...unfortunately, as their name would suggest, the Carnivorgs are vicious carnivorous cyborgs who harvest the brain matter from the people they capture.
Also from Reynolds: In Chasm City, an alien marooned on Yellowstone is the only source of a drug that can suppress the melding plague.
Inverted -- or maybe not -- in one of the Sonja Blue novels, in which demons use the corpses of especially-evil humans as hard drugs. One demon gets bribed with some of Hitler's teeth.
Animorphs: The Venber are a sentient race with unusual physical properties, chief among them that if they are brought to a temperature above freezing, they melt. The resulting liquid is apparently an excellent coolant fluid for supercomputers, and the Venber were hunted to the brink of extinction for it.
Triffids in The Day of the Triffids are initially culled because their predatory habits pose a threat to humans, but when it turns out they can be exploited as a source of a high quality oil, they are captured, have their stingers removed, and farmed instead.
One of the Star Wars Expanded Universe novels had a spider monster that produced a spice called glitterstim, which needs to be harvested in complete darkness. The spider uses it to make its webs, while other creatures use it for some kind of mind reading.
Star's Tears, from "We Who Stole The Dream", a James Tiptree Jr. short story.
The Red Court from The Dresden Files, whose saliva is addictive and a fairly powerful narcotic, use this to hold onto political power in Latin America.
At the end of The Relic, the monster-creating reovirus is used by one of the survivors to concoct a new street drug, Glaze. It turns out to have some nasty side effects in the sequel, and its derivatives are even worse.
Inverted in Children of Earth, in which the 456 want our children so that they can get high off them.
In an episode a small group of humans exploit a Space Whale with a Healing Factor stranded on Earth; they use it for a cheap source of meat to wholesale.
In the Doctor Who story "Nightmare of Eden", part of the plot involves the spread of a new addictive drug, and another part of the plot involves a pack of alien monsters roaming around after escaping while being transported by a zoologist. It turns out that the zoologist is the kingpin of the drug operation, and the drug itself is derived from the alien monsters.
The 10th Kingdom has rather literal fairy dust, as in the dead remnants of a fairy, which is recreationally snorted by trolls.
In Lexx, Kai was animated by "protoblood", a secretion from the last of the Insects. Re-animated assassins like him were part of how His Divine Shadow maintained his tyrannical grip on power.
On Forever Knight, a vampire doctor used injections of her own blood as a "miracle youth-restoring treatment" marketed to aging rich people.
Vampire dust in the short-lived Blade TV series is another example.
Sam on Supernatural did this a bunch of times with demon blood. Sam used it during the season 4 arc to enhance his psychic capabilities to be able to remove a demon possessing a victim without harming the host. He then becomes somewhat addicted to the substance, even keeping a victim possessed so that he can "bulk up" for the final fight.
In The Sarah Jane Adventures first episode the Bane Mother's excretions are the main ingredient in Bubble Shock cola "It's organic!"
One Shadowrun supplement's shadowtalk includes posts by a sicko who'd kept an Awakened leopard with Healing Factor captive for years, periodically skinning it alive and selling the pelts. The same poster speculated about the possibility of catching a giant regenerating species of shark and selling its meat over and over again.
Dwarven settlements in the Forgotten Realms sometimes keep captive deepspawn as a food source, feeding them livestock so the aberrations will make numerous copies of meat animals. This can easily backfire on the deepspawn-keepers, if one of their captive monsters ever manages to sink its teeth into something more dangerous than cattle.
In Chrono Trigger, the Kingdom of Zeal used Lavos as a power source once they discovered it, instead of the sun energy they had been safely using for years.
In the Vampire: The Masquerade – Bloodlines sidequest "Thinned Blood", Vandal Cleaver is revealed to have captured a fledgeling vampire and kept her restrained to leech off her blood.
Bioshock: The little sisters, creepy shells of the children they once were, are organic factories of ADAM, the substance that powers the gene-manipulating plasmids, drug of choice for the Splicers that inhabit Rapture. Even the player has a choice to harvest them for a bigger payday instead of rescuing them.
In Futurama the favorite soft drink is Slurm, which is secretly 100% slug juice. As in, it comes from a queen slug. She and her underlings have built a powerhouse corporation by marketing this highly-addictive beverage.
Batman Beyond: Bane's blood is used as a steroid (after so many years of use, he'd become a wheelchair-bound cripple).
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