My players have recently fallen in love with a kobold arcane trickster.  They hired a half-orc cleric for some NPC healing and combat support and got his kobold sidekick free of charge.  The two are disciples of a heretical god and live on the margins of society, camping outside towns or sneaking into barns of lofts to sleep.  They are lice-ridden and smell badly.  The kobold speaks and understands common but he usually just communicates through the half orc as they have their own secret language of hand signals.  The few times the kobold speaks it is always to great effect.

Example:  The group had just defeated some army deserters who had taken a small village and run off its inhabitants.  In the process, they set the local in on fire to drive the enemy out, not realizing it was full of alchemists fire the deserters had stolen on their way out of the army.  Que massive explosion.  Later when they meet the inhabitants of the town and are receiving their heroes thanks and praises, a kindly old couple with canes and glasses walks up and inquires about the condition of the inn?  "The bandits didn't damage it too much did they?  It been in the family for generations."  While the players stammer over how to break the news to this sweet old couple, the kobold pops up from the back of the wagon, laughing evilly "Ba-BOOOOOOOOOM!" (then makes descending jazz hands to simulate burning debris falling back to earth).

I also treat him kind of like a squirrel or a dog... whenever there are other kobolds in the woods or somewhere nearby as they are travelling, he pops up and starts barking in goblin or koboldese and making super aggressive and offensive remarks and hand gestures.

It can take a few minutes to get the game back on track afterwards because the players thinks its hilarious. We have a very dark theme so I think he works to introduce some laughs without getting too out-of-game.

Idea: A shape-shifter who can't control when or what they change into.

I cannot get enough of this thread

In my campaigns there is always a special rock that can turn into a rock golem (i made the stats), but they always find the rock in very mysterious places. One time, they found it in there our rogues head and had to perform surgery to take it out

I have a pair of characters in my campaign which are a bard and the tavernkeeper. The tavernkeeper is a dwarf, and the bard subjects him to so many short jokes and such that he is officially immune to vicious mockery, because he's heard them all before.

I have one NPC that only speaks in riddles and rhymes. My players' 5-person party with a collective Intelligence Score of 12 freak out whenever he appears because none of their characters can understand what the heck they're saying.

I had a chicken farmer named Old Mac Donald.  He would tell the PCs (with increasing madness in his voice) "You might think raising chickens is all fun and games, but it's not.  It's a cluck-cluck here, and a cluck-cluck there, here a cluck, there a cluck, EVERYWHERE A CLUCK-CLUCK!!!!   Eee-aai-ee-aai-oh!"  And he calms himself for a moment.  "Sometimes, I think I need to quit this job before I go a bit peculiar."

I had a retired pirate with a parrot on his shoulder...  and a stream of bird poop dripping all the way down his back.  He also had a mullet.  This pirate was all business in the front, but that was no party in the back.  The bird's name was the Dread Parrot Roberts.

BOBLIN THE GOBLIN.

Man I did the same thing, its super fun.

Mine was a gnome called Ted Wandelcroft whos just been missing something since his elven wife left him for a goliath ("She said size doesnt matter!"). 

He offers to join the adventuring party before going slightly off and running off into the bushes for his own adventurers, maybe helping the party later as well

An NPC who is always standing on the same spot doing nothing until spoken to. Alternatively, they may be walking back and forth in a predictable fashion. They also say the exact same thing every time they're spoken to, never acknowledging whatever you tell them.

Quote from critterfreak >>

An idea that an old group i was with had was having an NPC who would appear every session to help the group but would also die every session. We stated that he had a undying curse where he would be fated to die everyday but be resurrected the next day. It was a fun recurring character that gave us alot of laughs. Hope this helps and i wish you luck.

Was his name Kenny?

Quote from IanBarus >>

An Imp that was too weird he was kicked out of the Abyss and now is trying to find a new master, which could be any member of the party. Snagull the Imp was a big hit in my campaign, especially with the annoying high pitched voice I did.

Considering imps are devils, I'm surprised he wasn't killed instead of exiled.