You win some, you ooze some
What are oozes?
Oozes are amorphous creatures that can change shape at will being made of a jelly-like oozy substance. They have very low intellect and primarily function on instinct alone.
They are blind but use a sense known as blindsight to allow them to sense nearby creatures and other things. Many believe that oozes were created by the demon lord Juiblex, though their true origins are unknown. Any association with Juiblex may be due to his physical similarities with oozes as well as his ability to control them.
Most oozes prefer dark and damp locations to dwell. This makes underground caverns and sewers ideal locations for oozes. Wizards will sometimes create them as guardians and test subjects to protect their lair.
Ooze abilities
Wile oozes are creatures of simple needs, their physical properties are what make them interesting. Their ability to squeeze through almost any space makes them capable of surprise attacks.
Oozes are also able to reproduce by splitting themselves. This attribute can make some combat encounters interesting as a slashing or lightning attack can help facilitate this process, causing an unsuspecting party to be fighting extra oozes.
Oozes are not like most creatures, and their very bodies can often have terrifying effects. Some will melt the very flesh off your bones with their acidity. Others can destroy the metal of your weapons and armour. Some also like to consume their victims in their gelatinous bodies, suffocating them while their flesh melts!
Types of Oozes
Below are all the types of oozes found in the D&D universe. There aren’t a lot of them, but with the release of Spelljammer: Adventures in Space, there is now an ooze that is a playable race known as a plasmoid:

Black Pudding
CR 4
A black, oozy creature that captures its prey by smothering them and grappling them in dark locations.

Gelatinous Cube
CR 2
A transparent, cube-shaped ooze capable of fitting any shape it needs.

Gray Ooze
CR 1/2
A gray ooze resembling wet rock or stone.

Oblex
CR 5-10
Extremely intelligent oozes created by mind flayers that feed on the memories of others.

Ochre Jelly
CR 2
A golden sludgy creature that lurks on floors and ceilings of caves and ruins.

Plasmoid
CR 1/4-3
Intelligent oozes capable of taking on humanoid forms and humanoid levels of intellect. They are also a playable race in DnD 5e.

Slithering Tracker
CR 3
A creature turned into an ooze in an unholy ritual known to hags, liches and priests of certain gods.
Using oozes in your campaigns
Oozes are excellent for causing unexpected effects on adventurers. This is a great way to cause havoc among adventurers, especially when combined with other creatures.
For example, you can throw gray oozes at a party to damage and weaken their weapons and armour as it corroded the metal. Ochre jellies will split into new versions of themselves if hit by slashing or lightning attacks, quickly overwhelming a party.
Oozes can make great traps and ambushes too. Gelatinous cubes are transparent and hard to spot. It’s easy to walk into one and become slowly digested. You could even have it so a trap set off will cause one to fall from the ceiling.
Slithering trackers also make great ambushers as they can hide in water. They can also suffocate their victims.
For something even more unique, you can use an oblex, a slimy creature that can steal the memories of its victims and reproduce a replica of that creature. It uses this replica to lure more unsuspecting souls into its grasp.