Clowns

 Out of all the underworld's vomit, there are none so hated as clowns. 

Clown

Long feet, bright red nose, sharp teeth, and long prehensile tongues.

Lvl 1-2  Def leather  Knife 1d6

Dex high  Dis silly, murderous

Schaedenfreude - Whenever one of their opponents rolls a critical failure, all of the clowns burst into laughter, gaining +1 to all of their d20 and damage rolls until the end of combat.  

Hidebehind - Clowns can hide behind each other to hide their numbers.  Up to 10 clowns can disguise themselves as a single clown.  The effect ends when combat begins, or when the clowns are observed from diametrically opposite points.

Sleight of Hand - Clowns seem to be unarmed, until they suddenly aren't.  They often approach enemies with their hands open, smiling.  (This is the most combat-relevant usage of Sleight of Hand.  Clowns can do other thiefy stuff with it, too.)

Clowns giggle and babble in mendax (a degenerate "language" that only ever produces gibberish).  They cannot speak common Gospeltongue.

Elder Clown

Lvl 3+  Def leather  Hammer 1d10

Dex high  Dis calculating, murderous

Elder clowns are recognizable by their ruffed collar and leathery frowns.  The giant hammers also help with identification.  For every 6 clowns encountered, 1 is an elder clown.  Unlike regular clowns, elder clowns are capable of human speech.  They are also capable of mimicry.

Mimicry - Elder clowns can perfectly imitate any sound that they have heard, even unreasonable ones (galloping horses, etc).

For every level above 2, an elder clown gains one elder clown ability (below).

Japery 1/day - The clown puts on a performative act.  (Miming, juggling, etc.)  All non-clowns who witness this must Save vs Charm or be fascinated.  Fascinating creatures get -4 to all rolls not involving the object of their fascination (for example, Defense rolls against non-japing clowns).

Dodge 1/day - Automatically avoid something that you could conceivably dodge physically.

Beckon at-will - as command "approach".

Balloon Animals - Can make balloon animals (Move slow, fly slow, Int 2).  Not capable of attacking, but can assist the clown in other ways.  Deal 1d4 poison damage when popped (5' range, no save).  Shows up with 1d6 balloon animals and can have as many as 6.  Balloon animals can track by scent and see invisible.

Whistle 1/day - Summons allies to the clowns side.  1d6: 1 lion, 2 elephant, 3 1d20 clowns, 4 random wandering monster (non-allied), 5-6 no response.

Infectious Laughter - Has an infectious bite that causes 1d4 damage, a swollen red nose, and uncontrollable laughter.  Uncontrollable laughter causes the victim to be unable to take any other actions, not even movement.  Cha save after each round of laughter to end.  Victim lapses back into laughter whenever they observe a natural 1 being rolled.  Their bite can spread the curse.  Cured by going to church.

from Killer Klowns from Outer Space (1988)

Juvenile Clown

Lvl 1  Def none  Bite 1d6

Crawl slow  Int 2  Dis violent animal

Juvenile clowns resemble clown-headed worms the size of children.

Clown Eggs

Worth 100s each.  A clown nest usually contains 1d6 eggs and 1d6 immature clowns.  The adult clowns are usually nearby.

If eaten, causes the eater to experience a pure, blissful sense of joy.  Laughter is quick, and fond memories are easy to recall.  

Religion

Clowns worship OmO, the false god of wisdom and foolishness.  The symbol of OmO is a nonsensical squiggle, never drawn the same way twice.  Sermons are conducted by fools and drunkards who do not know that they are preaching.  Ceremonies are held (secretly and illegally) in the temples of other gods.

Falsehoods are true, and Truth is a lie.

How to Use Clowns in your Campaign

Clowns are creatures of the underworld, like demons and slaad.  A bit more mundane and low-level, but they share the motif.

They also fit into the category of "creepy rogue-thief monsters" which is always a category that can use more members.  Let them be the creepy, sneaky, cackling, violent psychopaths that we all want clowns to be, with floppy wet shoes and stained white pantaloons.