Creating new monsters to throw at marauding murder hobos has long been a challenge for dungeon keepers. Collections of books have been put into writing, entire archives of monsters exist for this sole purpose, and any DK worth their dice has a tattered book of home brew monsters. That still takes time and energy, a resource that can be tapped out by the most ardent of slaughter monkeys hunting for gold, XP, and magic items.
The Horn of Magic Power
The Horn of Magic Power is something you can put on almost any pre-existing creature and turn it from mundane into magical. Once the mundane creature has access to the power of magic, it can talk, it becomes dangerous and intelligent, and it's spell like abilities only have to stick together thematically and require only the smallest level of justification.
Example: Camel + Horn of Magic Power = Camelocorn
The Camelocorn is a denizen of the deserts, the strange offspring of bestial djinn and camels. They love riddles, insults, spitting at people and generally being ornery. On the magic side, they can create food and drink once a day (usually for themselves), summon sand devils and imps, cast spells of protection from the elements on others, and according to legend, elder camelocorns can grant a single magical wish, but they are under no compulsion to do so, as they can only do this once, and afterwards they either shed their horn, or they die.
The Power of Improvisation
As a dungeon keeper and story teller, you are likely already well versed with the contents of the Monstrous Manuals and the other sundry bestiaries. The same goes for the omnicide commandos sitting at the table. With the Horn of Magic Power, any mundane animal can become an entirely new threat, one that they might not be prepared for.
Mouse + Horn of Magic Power = Shrink Rat
The Shrink Rat has two powers, it can make itself human sized, or it can make whatever it is chasing rat sized.
Snake + Horn of Magic Power = Striking Serpent
The Striking Serpent can fly through the air, it has acidic venom it can spit 20 feet, and it can hypnotize threats into leaving it alone
Goat + Horn of Magic Power = Baphomet Goat
The Horn of Magic Power doesn't make a three horned goat, that would be silly, it grants the common goat magic power, such as Dispel Good, Protection from Good, Smite Good, Curse, and other suitably evil things.
There are limits to what the Horn is effective on. The Horn has no effect on creatures that are already innately magical, thus putting a horn of magic power on a unicorn does nothing. Likewise, the horn has no power over already fantastical creatures that lack specific magic powers, thus putting a horn on a chimera, a golem, or some other creature will have no effect. (Because that's just getting absurd) Finally, the creature in question must be able to use said item, thus cannot be used by the incorporeal, those without fixed form (oozes, slimes, and jellies) elementals, and so forth.
The Horn as a Magic Item
The Horn of Magic Power can be a stand alone item, and functions as a wand. As such, whatever spellcaster that uses it will have strong advantages added to their spellcraft, though typically limited to brute force augmentation and not greater understanding. It can't help you learn a new spell, but it can take one you already know and supercharge it.
Authotr's Note - A silly item, but a theme I noticed in an idle click monster game, a number of mundane creatures appear with unicorn horns for no apparent reason. Thus, the Horn of Magic Power was born.