While people might remember it fondly, in reality, CatDog was a creepy show. It was one of the most bizarre '90s cartoons ever created and considering the existence of Rocko's Modern Life, that's saying something. Some of the reasons that CatDog was super weird are inherent in the concept - a cat and a dog attached at the stomach is unnatural even before you start thinking about the implications. Those implications are pretty terrifying, though. For example, how do the siblings go to the bathroom? No one knows for sure but there's no possible explanation that isn't horrific.
When re-watching any old episodes from this show, further weird details emerge. Another odd fact, Cat and Dog's sarcastic roommate, Winslow, actually raised them from childhood. If that doesn't sound strange, then you're not remembering all the episodes when Winslow and Cat actively tried to murder each other. Oh, and there's also an episode when Dog hallucinates that raw meat is talking to him. He ends up trying to cook and eat Cat. You probably won't find that episode in any Nickelodeon fan art.
There's No Obvious Way For Them To Use The Bathroom
One of the biggest CatDog mysteries is how they go to the bathroom. People have come up with plenty of theories and all of them are terrible. One involves excreting waste through each other's mouths in a reverse centipede sort of situation. One involves their excrement being teleported to another dimension and one involves having separate anuses and most likely, separate penises as well.
Creator of the characters Peter Hannan says about their situation:
"No one knows [how they relieve themselves], myself included. What I used to say, because I was worried that kids might think CatDog would explode if they didn't go to the bathroom, was that they definitely do go to the bathroom, but no one knows how. Nobody ever asks how Mickey Mouse goes to the bathroom, so we don't really need to know."
Mickey Mouse presumably has the parts required for the process, though. We don't see them because he's wearing pants. He doesn't have another animal's head attached to his rear end. So it's probably a different sort of situation.
In One Episode, A Tree Grows From Dog's Head
In episode "Dog's Strange Condition," Dog eats too much pecan pie and somehow a tree sprouts from the top of his head as a result. This is both physically impossible and visually disturbing but what's worse is how Cat responds to it. Cat is reassuring when his brother holds a chainsaw to chop the tree. Then the feline twin mocks him relentlessly.
Though he's taunted himself, Cat can't summon up any empathy for Dog. You'd think he'd at least be worried about the implications for his own body, since they're conjoined but apparently not.
They Spend An Alarming Amount Of Time Inside Each Other's Mouths
Cat and Dog spend a disturbing amount of time just hanging out inside each other's mouths. In one iconic clip, Winslow walks in on the two of them standing in their room while Cat is rooting around inside Dog's mouth. In another much more terrifying scene, Cat crawls through their body, leaves his skin behind, and comes out through Dog's mouth as a horrifying skinless monster.
He does this in order to forcibly brush Dog's teeth.
They Don't Seem To Share An Immune System
In episode "Sneezie Dog," Dog mistakenly believes that he's allergic to cats (that's kind of a huge problem since he spends every waking moment attached to one). As it turns out, he's actually allergic to his rodent roommate, Winslow. This is a classic trope straight out of The Brady Bunch and it's pretty hilarious if you ignore all of the snot-based humor. The episode seems like pretty standard fare for a kids cartoon but it has some serious implications.
An allergic reaction is basically the immune system overreacting to something harmless, like pollen or animal hair. In the episode, only Dog has any kind of allergic reaction at all; Cat is totally fine the whole time. This implies that despite sharing a body, the two of them have separate immune systems.
This so messed up because theoretically one immune system could easily see the other's body as a threat. The reaction most likely would not be sneezing, it could result in the rejection that occurs when a body receives an unsatisfactory organ transplant. Basically, Dog's immune system could totally destroy Cat's entire body, or vice versa. This might be avoided with immunosuppressant drugs but it's still a huge risk.