Fake Rabies

Uh oh, Fido's gotten into the whipped cream! Or the shaving cream, or the soap suds, or the toothpaste—the point is, white foam is dripping from his mouth. Cue people running away and screaming, "Mad dog! Mad dog!"

Sometimes it's a human being rather than a dog inspiring the panic.

Comic Strips

In Garfield, Garfield and Odie have had fun with Jon's shaving cream.
In Calvin and Hobbes, Calvin made an attempt to fool his dad into thinking he was rabid using toothpaste foam.
One comic strip in Viz ran a story about the then recent opening of the Channel Tunnel. An apparently rabid French dog emerged on the English side, and the main character exclaims "Oh no! In a million to one scenario, a rabid French dog has made it through the Channel Tunnel!" The dog is shortly followed by its owner who explains that far from being rabid, it ate some soap, which it had mistaken for cheese because we do not have soap in France.
There was a depressing Between the Lines strip where a dog had just finished eating some honey offered to him by a cat. The owners then think he's rabid, and the last panel shows the dog's tombstone as the cat smirks smugly.

Film-- Animation

In Over the Hedge, R.J. uses whipped cream on Hammy to scare off some Girl Scouts and get their cookies.

Film -- Live Action

Buster Keaton runs frantically from a dog that ate a cream pie in "The Scarecrow."
Roy Neary pretends to do this in Close Encounters of the Third Kind. He's brushing his teeth knowing his kids are behind him with a paddle and a polaroid camera. When the kids whap him in the ass he whirls around with a mouth full of toothpaste froth and growls "ARRGH," and they get the perfect shot.
Alka-Seltzer tablets were used this way for the purpose of "intimidation" in Little Giants.

Live Action Television

In one episode of Mystery Science Theater 3000, Tom and Crow find Mike passed out with foam around his mouth and begin treating him for rabies (the movie they'd been watching had to this point entirely been about the main character seeming to contract rabies). It turns out Mike simply fell asleep while eating a creampuff.

Stand-up Comedy

Bill Cosby's stand-up routine "Roland and the Rollercoaster" deals with his crazy friend Roland, who used to do things like keep soap chips in his mouth. When people think Roland's head has been turned around by the roller coaster (due to Roland wearing his clothes backwards), there is a throwaway line from someone seeing Roland: "And he's got the rabies!" It's a good Brick Joke as it usually takes the audience a few moments to remember the soap chips in the mouth (which was mentioned early on in a fairly long story).

Western Animation

In the the classic Disney short "The Mad Dog", Pluto escapes while Mickey is brushing his teeth, causing panic throughout the neighborhood.
In "Feed the Kitty", Marc Anthony pretends to be rabid to scare off his owner in order to save Pussyfoot, but she just scolds him for wasting her whipped cream.
In the Droopy short "Wags to Riches," Spike puts shaving cream on a sleeping Droopy and phones in a report of a mad dog, but a fan blows the foam onto Spike's face just as the dog catcher arrives.
In the pilot arc of Chip 'n Dale Rescue Rangers, Fat Cat gets Plato out of the way at one point by spraying his mouth with whipped cream and stuffing a visitor's lapdog into the middle of the mess.
Subverted by Freakazoid!'s dog Foamy, who really is rabid but Freakazoid doesn't seem to notice.
On Kim Possible, Ron makes himself a mascot costume for the Middleton Mad Dogs, complete with foam dispensers hidden in the head. "Mad Dog foams at the mouth!"
On The Penguins of Madagascar, the penguins decide We Need a Distraction when their friend Max the cat is being chased by Officer X of Animal Control, and use cream and fake ears to disguise Mort the mouse lemur as a rabid chihuahua. It works fine until he eats the cream. "I like diseases!"