WMG / CatDog

Winslow is immortal.
In The Great Parent Mystery, he was an adult when he found the baby CatDog. Not to mention that he managed to live 1,000 years just by bodybuilding in the Bad Future episode.
  • The future Winslow was supposed to be his descendant, I think...
    • Yes, the Winslow from Catdog 3001 introduced himself as Winslow the 38th, plus the narration specifically stated that ALL of his descendants gradually got buffer and buffer thanks to the near constant training they all did.

CatDog are\is a Time Lord.
Once there was a Prydonian Gallifreyan named Canisfelishivolar, or "Canfell" for short. When Canfell turned 8, he looked into the Eye of Harmony, and got enrolled at the Academy. The day right after Canfell got enrolled at the academy, he found a TARDIS and entered it as a stoveaway. As Canfell was a quite curious Gallifreyan, he jacked up quite a few commands, and as the TARDIS was already piloted by a twelfth regeneration Time Lord named Mr. Sunshine (who had already an adventure planned), Sunshine's TARDIS crashed in the planet CatDog is set in. The TARDIS crashed so hard, Mr. Sunshine lost all of his "time-lordiness" and fused with his old and now unusable TARDIS, while regenerating\transforming into a non-human-looking form, which is what we see today. Canfell also regenerated into his second regeneration, who, due to all the mutation from the TARDIS crash and Canfell himself being a rookie, regenerated into a form looking like a cat and a dog stitched to each other... but uh, he was ginger! Since he looked like a "baby" or little more than it, he was adopted by a monkey and a frog. Canfell was renamed some name such as Mark or George or whatever, and when he was around 4 or 5 (12 or 13 if you count his first incarnation) he got separated by his adoptive parents by a violent storm, found by an orphanothrophy, renamed himself "CatDog" after the animals he looked like, and thought the Gallifrey things were just from a cartoon show he had seen in his infancy, and that he just imagined them as real. CatDog grew other 21 (at least) years in his second incarnation, living as a native cat stitched to a native dog.

Rancid Rabbit's niece Rancine is really his daughter.
She was born out of wedlock; he just says she's his niece to cover up the fact (and to avoid giving himself a bad name).

CatDog are neither cat nor dog.
Despite their name, CatDog are not two separate creatures. In fact they are both the same species but have mutations that go beyond being conjoined at the hip, affecting their facial features so that each head resembles a canine and feline (kind of like how hyenas belong to their own family despite looking like cats/dogs). The reason why they act like their species' stereotype is due to social expectations, and lack of knowledge of their true origins. Further evidence for this comes from Mr. Sunshine, who doesn't fit in any category of recognizable species (human head, green skin, pig-like tail, etc.) It's likely CatDog don't belong in any of those categories either, but their physical features make them similar enough to cats and dogs that both they and other people think they really are cat and dog.

Going by the above, their adoptive parents are actually their birth parents
The reason they didn't tell them this is because 1. they feared that CatDog wouldn't believe them, and 2. even if they did, they'd still need to explain the confusing issue of how exactly they managed to conceive.

Dog has nine lives through Cat
The reason Cat thinks Dog only has one is because neither of them know this (the only way they would would be for Dog to die, and of course they're not going to let that happen), so he simply assumes that Dog only has one life like all other dogs. Since they share a body, however, Dog actualy has nine lives like Cat. Dog isn't as organized as Cat, either, so his eight spare lives are probably just sitting there waiting to switch out. Hey, it's better than the alternative...

Dog will get very lucky and befriend a moe anime girl someday.
And it will annoy Cat to no end.

Cat eats a lot of breath-mints.
The only way they can go to the bathroom is through ejecting the waste through hairballs. They're probably used to it, but it'd still make their breath stink to high heaven. To counter this, Cat has to gargle down metric craploads of breath spray, mints and toothpaste for good dental hygiene.

Mervis and Dunglap are step-brothers.
Think about it. Home come these two constantly hang out with each other despite always getting into a fight over everything? Because their parents make them do so in an attempt to get them to bond as brothers. Them being step-brothers is justified by how the former is a pig and the latter is a weasel.
  • Supported by the fact that the two appear to live together.

Catdog was created by Dr.Weird from Aqua Teen Hunger Force
They are an escaped lab experiment who were adopted.

CatDog's biological parents abandoned them because conjoined twins normally die shortly after birth.
The heartbroken cat and dog couple didn't want to watch their babies die, and left them in a basket outside a cave so they can pass in their sleep away from them.