Plenty of enemies in games drop items when they are defeated.
Sometimes, this can present a bit of Fridge Logic as to why the enemies have those items. What exactly are those slugs doing with a sword and tunic? And how do monsters have ammo to help you reload weapons (which they obviously never use), but not ammo for weapons you haven't found yet? Furthermore, monsters will tend to drop items associated with their specific abilities. For example, a monster with a petrifying gaze will commonly drop a de-petrification potion, implying that the monsters are actually made of the stuff, in convenient, easy-to-use form.
A potential way to explain this is that killing these monsters is obviously the popular thing to do and somehow they aren't all extinct. Maybe that's because some people aren't as good at it and got eaten along with their stuff. Perhaps they just didn't use that de-petrification potion in time. This doesn't explain how an enemy could have some items that should be readily visible, like in the page image.
Compare Vendor Trash for when enemies drop items that you can't use, but that can be sold for ones that you can. Compare Money Spider for animals dropping money. Also see Randomly Drops.
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In the Zelda series, most enemies (and random objects like pots or bushes) drop rupees, arrows, bombs, magic potion vials, and hearts at random. Even better, whenever you get a new item (bow, bomb bag, slingshot, etc...) that consumes something, whatever it is suddenly starts appearing everywhere in spite of its not showing up before.
In The Legend of Zelda: A Link to the Past, there's no bomb bag, and bombs start dropping from pots or random monsters as soon as you leave the Sanctuary. Arrows, too, though they're useless until you get the bow.
A variant of this occurs in the Monster Hunter series; one example is that you can carve more than one Crooked Horn from the horn you just broke off Ceadeus.
First Person Shooter
In Borderlands, it's not an uncommon occurrence to blow up an unarmed midget, only to see him cough up a rocket launcher that's larger than he is.
In the World of Warcraft MMORPG, most non-humanoid opponents drop items instead of money. While there is some attempt to make the items dropped match the creatures in question, it is often forced, such as making the bodies of most types of carnivorous animals - including things such as harpies and giant spiders - edible delicacies and/or requisite components for items the players can make or trade for. These are often also Plot Coupons for one or more quests as well. Even so, it is not unusual for a deceased opponent to leave behind something that makes no sense at all for them to have had.
Raid boss class enemies, however, typically hoard both gold and 2 to 6 pieces of equipment (out of a total loot table of 8-12 specific items), regardless of what they are. Sometimes the equipment is mildly appropriate, such as a weapon the enemy was seen to use, a dragon's jawbone one may wear as a helmet, or something thematically linked to the enemy's lore. Most items, however, have no reason whatsoever to be upon this particular boss. One may wonder why exactly does Ragnaros, a massive fire elemental lord who has no legsnote In the incarnation when he dropped pants, to be specific. His later reappearance, now with legs, doesn't drop any pants, but instead shows off a collection of helmets and weaponry., have a vast collection of pants for every class in the game... Or not...
Items themselves also come up in improbable locations. One is left wondering not only how the item fit into the animal, but how the animal managed to kill and eat somebody who was using what is sometimes very good equipment, and why, if it did, it lost to you with your inferior equipment.
The most notable example, though, are the crafting recipes. Not only why would this wolf have eaten a tunic pattern, but how is it still readable after sitting in stomach acid for a day or two?
And how did that fish know to eat only the recipes it was an ingredient for?
It's also sometimes goes backwards with an impossible drop for all the wrong reasons, such as all those basalisk wandering the Blasted Lands without a brain, or those raptors without claws or teeth...
With a bit of thinking, this can sometimes make sense. There are quests which are essentially about the player getting some stuff so the quest-giver can make a potion with it, make boots out of it, or cook it or something like that. Stabbing a basilisk through its brain with a poisoned knife, or burning it, or doing otherwise nasty things to it makes it unfit for cooking (not that there is a logical connection between using poison and getting edible things).
Worse is that sometimes quest items drop for all the party members. This might make sense if it's about claws from four-legged best for four players (not that a single player can take all four claws at once), and even a raptor might be carrying multiple heads of its kind for whatever reason, but it becomes truly impossible when a named NPC drops its head five times, with it being shown having only one.
Averted in the non-interactive "game" Progress Quest which has most monsters drop items specific to the monster. Some monsters and any (simulated) wandering adventurers you run across drop special items conforming to the template adjectiveconcrete noun of noun, usually abstract. When your character's Encumbrance capacity is reached, he heads to the market to sell it all off and buy weapons and armor.
In the MMORPG Dream Of Mirror Online no enemy will ever inexplicably drop gold, but will often drop items whose sole purpose is to be sold at a set price to NPCs. Some of those drops are even more inexplicable than the gold they replace however... Like pigs carrying carved wooden sculptures of bears, birds with perfume, and eventually male human wizards who drop ladies underwear.
EverQuest II features loot that drops in the form of treasure chests. The type of chest that drops determines the value of the treasure. There are small wooden chests, normal treasure chests, Ornate chests, and the absolutely gigantic Master Chests (which are larger than the majority of the smaller player races, who stand between 2 and 4 feet tall.) Monsters in the game can be as tiny as will-o-wisps or Brownies, which only stand about 2 inches off the ground. At that size, even a small chest will completely crush the corpse of the monster you just killed. How a 2 inch tall Brownie can carry around a small bank vault like a Master Chest is anyone's guess.
Everquest is no stranger to this trope either, with some interesting twists. For example, pickpocketing a foe allows a rogue to take an item off the creature's loot table, leading to such bizarre occurrences as pickpocketing a Dwarf and stealing a watermelon from him (how did he not notice that?) or pickpocketing a goblin and stealing his brain.
Un-averted with the Pickpocket skill, which lets you steal a possible drop... like for instance the monster's skin. Word Of God explanation: it just happened to be carrying around a skin belonging to a different monster of the same species. Eeeek.
The GameInformPowerDailyPro dungeon has randomly-generated drops, so you can get such illogical things as turtles dropping flaming swords. Lampshaded by the roasted chicken drop—"not the sort of food you would expect to find being carried around by some random monster"—a parody of the inexplicable roasted chickens/turkeys in Duke Nukem II.
All over the damn place in Runescape. Some of the drops make sense, like many things drop bones, hides, the goblins drop goblin armor, etc. But then you have giant roaches that potentially drop ancient Dragonkin artifacts, dragons that have chocolate cakes, and the like. And that's before you take into account the Rare Drop Table, so you have situations where monsters like the abyssal demons can drop five hundred sharks (in banknote form).
Ragnarok Online has this in spades. From cute little baby pigs that somehow drop heavy axes despite not even being capable of downing a first-class character to bird eggs dropping china platters. Surprisingly, the entire game averts Money Spider.
When Susan-oo killed the serpent Orochi; the legendary "Grass-cutting Sword" Kusanagi was found in its tail. (Some modern writers speculate that it was stuck in the Orochi from a previous hero's unsuccessful attempt at killing it; but this is never stated in the legend.)
Real Time Strategy
The enemies in Warhammer 40,000Dawn of War 2 randomly drop various articles of Space Marine weaponry, armor, attribute-enhancing Purity Seals and other stuff. While it could be justified for the Orks, who are notable plunderers and looters, and even for the Eldar who might just happen to be carrying these things back to their base to study, but it is entirely confusing for the Tyranids, who have no need for such things and no means to carry them. And there is still a question of why and, most importantly, how would they lug around armor plates from a Mini Mecha Dreadnought?
Tyranids eat literally everything, and have no internal digestive system — they instead leap into digestion pools created by Tyrannoforming so the Hive Fleet can reclaim the raw materials. Presumably, the items they drop are whatever made it through being eaten intact enough to salvage.
It was then Hand Waved as being "released from the Blood Raven vaults" as reward instead. But the question of how some of these items reached the chapter vaults in the first place led to the Bloody Magpiesmeme.
Mostly averted in the earlier Warhammer Dark Omen. Your enemies are humanoids or, occasionally, huge monster spiders/scorpions, so if they drop a treasure chest or a potion now and then, it doesn't look too conspicuous. Moreover, if an enemy group carries an artifact (like a banner that invokes lighting bolts), they will actually have sense to use this artifact against you! And every enemy keeps their eyes open for some unattended goodies and will not hesitate to pocket them.
This trope is used by several different enemies in Diablo, but the most egregious example is the Swarms: swarms of insects able to drop items like pieces of armor.
Path of Exile, being a Spiritual Successor of Diablo II, also suffers from this. Humanoid enemies dropping weapons and armor is all well and good but then you have giant spiders or animated streamers of cloth that will drop sets of plate armor.
Nethack partly averts this; many dropped items are physically in the monster's inventory (or is the monster's corpse itself). If an orc swings at you with a long sword, he'll drop that same long sword when he dies. Some items are generated upon death, but the game checks the size of the item to prevent impossible situations like killer bees dropping plate mail.
Everquest: Champions of Norrath on the PlayStation 2 features Fire Beetles, which only stand about half a foot tall, but can end up dropping longbows, swords, giant war mauls, and various forms of armor along with gold.
The RPG Mass Effect 1 has a somewhat specific variation of this effect- since the game is completely devoid of standard Vendor Trash, all recovered items must take the form of weapons, armor, tools/implants, and upgrade modules for the aforementioned weapons and armor. This can lead to a seemingly odd proliferation of military-grade equipment in the world. While it is perfectly reasonable to recover a Scram Rail or High Explosive Rounds from a krogan mercenary, it is odd to recover assault rifles from apparently naked and weaponless cyber-zombies, and advanced ultra-tech materials from lost, 60's era Soviet lunar probes.
The Rune Factory series averts this, with monsters dropping items they might plausibly have, such as a shrunken head dropped from a Gnome chief.
Knights of the Old Republic featured enemies who would drop random weapons and armor. While this made sense, much like Mass Effect, there were peculiar instances. It was entirely possible for enemies roughly the size of a large dog to drop six-foot long, double-bladed swords.
The sequel had a semi-justified example in Cannocks, a creature said to eat literally anything.
Averted in Gothic where the drops make almost total sense. If a humanoid NPC has a weapon in his hand at the moment of his death, he'll drop it - the player can pick it up and then go through the body's inventory, picking and choosing the best loot. Non-human monsters don't initially have a visible inventory; the player has to learn specific hunting skills in order to, for example, skin wolves for their pelts (which can then be sold to traders).
Is it that unlikely that a wolf ate a traveling thief who couldn't fight it off?
Similarly averted in Spiderweb Software's Geneforge games; with one justified straight example. Monsters do not drop anything. Searching a monster's nest can turn up random items; ranging from useful equipment, to Vendor Trash, to worthless trash. (Monsters pretty much collect anything shiny — from shiny coins, to shiny swords, to shiny rocks — as well as useful things like clothing to pad their nests with.) Corpses of people can be looted for useful items and vendor trash. The only other locations to find stuff are storage chests and jars located in and around buildings and settlements; which randomly contain some combination of useful items, quest items, vendor trash, and actual trash. (The random chest popping up in the middle of nowhere is also avoided.)
The one straight example is the "thorn baton" short-range ranged weapon. Ammo for this weapon, thorns, literally grow on bushes scattered around settlements. Justified in that both the weapon and the ammo-growing-bushes have been bioengineered by the Shapers, a game faction whose hat is biotechnology.
"Turrets", bioengineered sentinel gun-creatures, also use thorn ammo, and drop it when killed.
In Titan Quest, everything dropped by monsters (except certain quest items and the enhancer items) is something that the monster that dropped it was using, this includes extremely powerful weapons and armor. Non humanoid monsters rarely, if ever, drop anything other than monster specific charm items.
Final Fantasy XII. The Humbaba monster drops a Beastlord Horn. Humbabas don't have horns.
In the Necrohol of Nabudis, the monster that drops the Maximillian armour is the wrong shape to wear it, and the monster that drops the Runeblade has no hands and kills its enemies by trampling them or casting spells.
Ultima VII and it's sequel come so close to averting this. Deer you kill drop legs of meat, perfectly reasonable. But. . . the (normal, four legged) deer tend to drop five legs of meat apiece for no apparent reason whatsoever.
Only humanoid monsters in Cute Knight drop armor items, which makes sense. However, dresses are also classed as armor items, which raises some questions about what a goblin was doing with a nice party dress.
Creepers in Minecraft normally drop gunpowder, which makes sense, but if they're killed by a stray arrow from a skeleton, they drop a music record. Guaranteed. Zombie Pigmen may drop Golden Helmets, despite the fact that they are never seen with the helmets on.
Zombies also have a rare chance of dropping carrots, potatoes, or iron ingots, which makes a bit more sense if the zombie is an infected villager.
Assassin's Creed: Brotherhood does this. Oh, sometimes it is reasonable, like guards holding crossbow bolts or bullets - Real Life soldiers do hold onto ammo - or Borgia messengers holding onto rare Vendor Trash that might well be what they are supposed to be transporting. However, when guards pack poison vials or the random pickpockets are also holding onto rare Vendor Trash, it gets less plausible.
It's even worse in Revelations, when bomb components are added. Why, exactly, would a halberd-wielding palace guard be carrying deadly poisonous datura powder?
Parodied in GURPS: Creatures of the Night which includes a completely immobile plant monster that comes complete with a treasure trove full of things that are useful when trying to kill plant monsters. Why? Because it enjoys murdering adventurers and taking their stuff (which it then buries somehow).
Dungeons & Dragons usually attempts to justify monster treasure in their Monster Manuals; the more savage varieties of monster tend to have the gear of previous attempts at killing it strewn in their lair, while more intelligent ones like how it looks. The really dumb or bizarre monsters don't have treasure listed for them at all.
Dragon Mango: Parodied; Mango receives a suit of fashion plate mail for swatting a mosquito, then wonders how killing a bug made armor appear. (Answer: it was a drop bug.) She later has to assure her mother that she didn't hack anyone for it.
Undertow came up with an interesting explanation on this page. The author's idea was the loot came out of the stomach of the monsters from unlucky adventurers they had eaten. Ew.