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 Wings of Wondrous Power
The Wings of Wondrous Power are 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 flight, it becomes much more difficult to fight, becomes potentially more valuable as a mount, and there is a 50% chance it gains magical powers, and a 25% chance it gains human level intelligence (if it didn't already have it)
Putting wings on mundane things to make them wondrous is old hat. Horse + wings = pegasus, human + wings = angel, Lizard + wings = quetzalcoatl or dragon. This being said, the dice rolling derelicts of death and their player characters are very likely to accept almost any creature with Wings of Wondrous Power strapped to it.
Beneficial - Cats with wings are likely to be wise in the ways of magic, know secret passages, and be deft at the hunting of imps and fairies. there is nothing more loyal than a winged hound, or more easily distracted.
Absurd - bison with wings? Making a buffalo wing joke? The genocidal looters won't be laughing when a sky stampede sends them running for cover, and when the avatar of the God of Sports Bars shows up wanting to know where his herd is, the PCs could find themselves with a dudely geas.
Evil - Spiders with wings! Scorpions with Wings, Tiny flesh eating babies with wings, flying zombies, flying monkeys, flying eyeballs, literally anything you hate or find scary, give it flight, and watch the carnage!
The Wings of Wondrous Power as a Magic Item
The Wings of Wondrous Power graft themselves permanently onto the wearer and can only be removed by use of a remove curse or dispel magic, or by their death. The wings will scale to fit and will give the wearer a flight speed that is double their walking speed and agility relative to their size, with a human having average agility. If the wings are equipped, the wearer gains access to the skill Flight, which can improve their agility. Strength based feats can be used to enhance the endurance for flight, or flight speed.
Author's Note - DungeonCo is a series of Silly magic items to be thrown template style on existing mundane creatures. Have fun with it.