Many people keep dogs as pets. Most people have toilets. These are jokes about dogs drinking from toilets, which is a common gross-out joke about dogs.
This can be Truth in Television if the dog is tall enough to reach the bowl (though of course, not everyone will find it funny, and sometimes other animals, such as cats, do it too), and the reason is likely because the water gets oxygenated by flushing, is cooler than their bowl because of the porcelain, and dogs don't know (and probably wouldn't care if they did) what we use it for.
See Urine Trouble, Yellow Snow (do note that this can apply to other animals too) and (in the cases where they pee on them) Dogs Love Fire Hydrants for other stock toilet jokes involving dogs and Flushing-Edge Interactivity, Swirlie, Goodies in the Toilets, and Fun with Flushing for other jokes about toilets. If an animal who is not a dog drinks from the toilet, that's usually an example of All Animals Are Dogs (though some animals like cats sometimes do it too). Sub-trope of Stock Animal Behavior.
- The Dog Man story "Tongue of Justice", Petey had brainwashed the police force with tampered air fresheners, and Dog Man was chasing him in his secret lab when he couldn't resist the urge to drink out of a nearby toilet. Petey then gloats that he hid the antidote to the brainwashing in the toilet, so Dog Man vomits up the toilet water, licks it up, and slobbers the brainwashed policemen with it.
- This◊ Far Side comic strip shows a dog TV presenter introducing a talk show segment about dogs who drink from the toilet bowl - the dogs who will be talking are seen silhouetted in the background.
- A common joke in Garfield:
- Mother Goose and Grimm has the dog Grimm host a poker night at Mother Goose's house. When Attila the cat wonders why the game has been set up in the bathroom, Grimm casually points to the commode and says, "Wet bar." Verily, one dog is slurping away at that moment.
- In The Simpsons fanfic Maggie's Toilet Training, Santa's Little Helper drinks from the toilet and Maggie copies him, then Homer copies them.
- In the My Little Pony: Friendship Is Magic fanfiction Pound and Pumpkin Cake's Adventures (and Misadventures) in Potty Training, when Pumpkin Cake is staying with Applejack and her family, Winona drinks out of the toilet.
- Barnyard: After the death of Ben, Otis' father, Duke the sheepdog holds an emergency meeting over who the new leader of the farm animals should be. When Duke nominates himself to be the new leader, Pig objects and tells him that he saw him drink out of a toilet once, to which Duke tells him that he only did that because his water bowl was empty.
- One lesson Mittens the cat gives to Bolt the dog during their travels cross-country is that the toilet bowl doubles as a water dish. Bolt is aghast and mildly squicked at this news.
- In Open Season 2, the Pet Paradise place for owners to leave their pets while on vacation has special toilets for dogs to drink out of.
- On The Secret Life of Pets, during the party at Pops' apartment, a dog is drinking from the toilet as others chant, "Chug! Chug! Chug!"
- The opening of the fourth Beethoven movie has one scene where Beethoven drinks from a toilet — then when he finishes and leaves, he accidentally pulls the toilet seat off and gets it hung around his neck (which is where the image on the film's cover comes from). This becomes an issue when Beth tries to use the toilet immediately afterward....
- Cats & Dogs invokes this by having spy dogs in training drink from toilets to hide the fact that they are actually receiving messages from HQ.
- One of the opening shots in Clerks is a shot of Dante's dog drinking from the toilet.
- The Flintstones: One scene has Dino, the Flintstones' pet snorkasaurus, drinking from a stone toilet.
- My Dog Skip: One scene features Skip drinking from the toilet, a scene prominently featured in advertisements.
- Discussed in Scooby-Doo. While investigating a scary ride, Scooby and Shaggy find themselves entangled on sausages with the only option being to eat their way out. After one bite, Scooby complains about it being plastic only for Shaggy to quip how Scooby drinks out of the toilet. Scooby's response to the statement is telling Shaggy that he does that too.
- The Smurfs, when Elroy the Beagle sticks his head in the toilet to look at the Smurfs who have fallen in there, his owner Grace, thinking he's trying to drink from it, says, "No, no!".
- Uncle Buck: When Cindy calls to check how things are going, the issue of providing water for the dog comes up. Buck replies that he's been leaving the toilet seats up.
Buck: Yeah. *beat* Is that what the blue water is? It's not good for him, huh? That might account for something... *beat* Nothing, just, uh... *clears throat* an odd coloration out on the grass.
- On The Tonight Show Starring Jimmy Fallon in 2015, Fallon shared a story about Wichita Falls Texas, which was having to recycle toilet water because of just how bad the 2011-2015 drought in the area had gotten. He followed it up with "Dogs everywhere have reacted to the news by asking 'You guys are just figuring this out now!?'"
- In Freefall, immediately after warning Tangent not to treat Florence like a dog, Naomi mentions the potable water system and the toilet with a phrasing that makes it sound like she expects Florence to drink from the toilet. 
- In Kevin & Kell there's a Running Gag that Rudy, the wolf teenager of the mixed species family, likes drinking from the toilet.
- Neopets mentions that it's against the rules for the Neopian Times stories to feature Neopets acting in a "demeaning manner" and gives drinking from toilets as an example.
- In one episode of Dexter's Laboratory, Dexter tries to inject himself inside a sick Ded Dee cure her cold, but ends up in the dog instead, which he doesn't notice. At one point, the dog is seen drinking water from the toilet, which makes Dexter believe Dee Dee is infected with a dog virus.
- The Fairly OddParents!: The episode "Dog's Day Afternoon" has Timmy swap bodies with Vicky's dog, Doidle. During one moment, Doidle (in Timmy's body) starts drinking out of a school toilet. Upon witnessing this, Chester and A.J. claim Timmy to be tougher than Francis, prompting the bully to start drinking out of a toilet as well, and then they join in afterward.
- In one episode of The Garfield Show where an alien switches Garfield and Odie's bodies, Arlene tries comforting Garfield by telling him to think about how many things he can do as a dog. The first things that come to his mind are chasing his tail and drinking from the toilet, which doesn't make him feel any better.
- Martha Speaks:
- In "Martha in Charge", Martha cites drinking from the toilet as a perk of doghood.
- In "Martha Runs the Store", Skits wanders into the ladies' bathroom at the mall and drinks from the toilet.
- In the unaired Rugrats pilot episode, "Tommy Pickles and the Great White Thing", Tommy is curious about the toilet, which Phil thinks is a giant water dish, since he saw Spike drinking out of it. Near the end of the episode, Spike comes into the bathroom and drinks out of the toilet.
- The intro to Tuff Puppy shows the protagonist drinking out of the toilet.