The dogs that attacked ThunderClan in the first arc are described as large black-and-tan dogs. Fan-works almost always depict them as Dobermans or Rottweilers, when they're not vague silhouettes running on Rule of Scary.
The graphic novels often features completely Off-Model designs that fans dismiss as non-canon. The sole exception is Scourge, or more specifically Scourge's collar as depicted in The Rise of Scourge. It's purple in the manga. Thus, his collar is usually purple in fan-works.
Many fan-artists depict clan leaders as having star-shaped marks on their head, similar to Tailchaser from Tailchaser's Song, called "leader marks". It's usually not a physical mark. Instead, only StarClan-worshipping cats can see the mark (which means non-Clan cats and non-religious Clan cats like Mothwing don't see it).
Kinkfur is described as having permanently messy fur. Many fans assume that is due to a rex coat that just looks messy to other cats.
It's commonplace in the fandom to depict cats with leaves and flowers behind one of their ears. Medicine cats, especially Spottedleaf, are particularly prone to this.
Dovewing's eyes are usually green, but they turnblue when she's using her powers, and occasionally they're portrayed as having been golden before she developed her powers. This is a result of her eyes having a bad case of Inconsistent Coloring in canon — they were initially portrayed as golden, were consistently depicted as blue throughout Omen Of The Stars with occasional appearances of green eyes, and later was consistently shown with green eyes post-Omen Of The Stars (with one mistake in Thunder And Shadow).
Longtail is nearly always depicted by fans as being a particular shade of pale yellow with black stripes, after SSS Warrior Cats portrayed him as such. In the books, he's a victim of Inconsistent Coloring, and is either brown or silver.
It's not uncommon to see cats being explicitly depicted as certain cat breeds, even when it's highly unrealistic. Popular examples include Norwegian forest cat Graystripe, Abyssinian Firestar, Persian Yellowfang, Maine Coon Tigerstar, and Russian Blue Bluestar.
Firestar as an Abyssinian is slightly more plausible than Clanborn cats, due to the fact that he was originally a kittypet, and that both his parents (Jake, a ginger tom, and Nutmeg, a brown-and-white molly) and his sister (Princess, a brown-and-white molly) have coat colors possible in the breed.
The books make it a point that Hawkfrost is the spitting image of Tigerstar, just with blue eyes and a white belly, but fanartists like to draw him as being a lynx point Siamese (due to his mother).
Nowhere in the books is it stated that Ivypool is Covered in Scars, but you'd be forgiven for thinking that's a canon fact by how common it is to see her portrayed with them in fanart, fanfics and MAPs. This also applies to a lesser extent to other Dark Forest trainees like Breezepelt.
Thanks to popular MAPs, Sleekwhisker is often presented with either a cross-shaped scar on her face or bangs.
Certain cats, like Squirrelflight and Ashfur, are commonly depicted with freckle-like markings.
Squirrelflight is often drawn with tabby stripes. She is predominantly red with only minor striping (because cats can't be orange but not tabby).
One growing trend amongst fanartists is to give the characters more genetically accurate designs, sometimes even ones that deviate greatly from canon. Common design variations include making Leafpool red or cream to match her parents, representing Blackstar's white body and black paws as him being a seal colorpoint, or making various characters tortoiseshell to make their close relatives' canon designs more plausible.
In more recent books, the Sisters are confirmed to be descended from kittypets and are much bigger than clan cats. Most fans agree that their ancestors likely include a larger cat breed, commonly thought to be either Norwegian Forest Cats or Maine Coons.
Violetshine's appearance is sometimes explained to be vitiligo, and that she will slowly lose pigment as she grows older. She is also often depicted with purple eyes in order to match her name.
Her sister Twigbranch, meanwhile, is often portrayed as tall and thin to go along with her name, and occasionally is shown as much taller than her mate, Finleap.
Her son Rootspring is also drawn as inheriting her purple eyes.
Related to the above, Heathertail is also commonly depicted with purple eyes (granted, this was canon before they were retconned into blue).
Shadowsight is occasionally interpreted as having red eyes.
He's also often depicted as either small and chubby or big and muscular in fanart, even after the books described him as scrawny.
He's also commonly depicted with a scar known as a Lichtenberg figure after being struck by lightning in Lost Stars.
Even though she's described as just gray in the books, fanart of Bristlefrost as a dilute tortie (gray and cream) because of her parents — Ivypool is gray and Fernsong is gold — is quite common.
Clear Sky is described as a solid pale gray in the books, but try finding fanart that doesn't depict him with white splashes or markings.
Before River came out, plenty of fanartists have depicted Flamepaw as a tortoiseshell or rusted black to match his name. Afterwards, such depictions have decreased, but still fill up a considerable amount of Flamepaw/Nightheart art.
Mothwing is never stated to be a large cat, yet it is very common for fanart to depict her as a Palette Swap of her father Tigerstar.
Meanwhile, Hawkfrost is stated to be large and muscular as his father, but a substantial number of fanartists draw him as lithe like his mother.
Neither Leafpool nor Squirrelflight are mentioned to be chubby, and Leafpool has actually been stated to be lithe, but there's plenty of fanart showing one or both cats as small and stout.
Dovewing and Ivypool are mentioned by Bristlekit in The Raging Storm to be similar-looking, only with Ivypool having white splashes. In fanart, their differences are far more distinct, Dovewing is often drawn as much shorter/smaller and chubbier than Ivypool, longhaired (or at least fluffier than Ivypool), and with no to minimal scarring. Ivypool, on the other hand, is frequently drawn as taller/larger and sleeker than Dovewing, shorthaired (or at least not as fluffy as Dovewing), and being covered in scars from her time training in the Dark Forest.
Other
Due to the series not always making it clear who is whose parents, fans take it upon themselves to come up with family trees. For example, one of the most commonly accepted theories is that Heathertail, Harestar, and Kestrelflight are siblings, due to them being apprenticed at the same time and looking alike (Heathertail is brown, Harestar is brown-and-white, and Kestrelflight is gray-and-white) and their parents are largely suspected to be Onestar and Whitetail.
More often than not, Jake is headcanoned as being bisexual. Canonically, he mated with two she-cats and seemed to genuinely love Quince, and he also has some extremely heavy subtext with Tallstar.
Warriors doesn't have any canonical transgender characters, but there are a few characters the fandom tends to agree are trans.
Redtail was previously stated to be the father of Sandstorm. As fertile tortoiseshell toms do exist but are very rare, he is instead occasionally depicted as a trans tom and the damnote The technical term for the parent that carries offspring of Sandstorm. Another less common idea seen with this is that Runningwind is his mate and the sire of Sandstorm, due in part to their close age, the two having no relation, and the fact that the two have very little characterization in canon.
Rowanclaw was originally depicted as a she-cat, but in every book after Dawn he's portrayed as a tomcat, and later goes on to have kittens with Tawnypelt. Many fans latched onto the idea that he's a trans tom.
Mothwing is frequently depicted as a trans she-cat. This is partially from a popular Fandom-Specific Plot where she takes Crowfeather's role in Leafpool's story.
Nightheart repeatedly shows discomfort with his original prefix "Flame" as it was chosen to honor his great-grandfather, and when he chooses a new prefix to honor Nightcloud, a female warrior, he's met with disbelief and derision from those around him. While almost certainly unintentional, the parallels to young trans people coming out has led many to view Nightheart as a trans woman.
There's the concept of "traditional names". These are the unspoken rules of what names can be used for Clan cats. In a case of First Installment Wins, the names from the first arc are used as the basis for the rules. Amongst the rules are that kits can only be named after their appearances and suffixes must relate to the character's appearance or accomplishments.
Snowkit survived and he is either hiding somewhere or he became Snowtuft.
For those who didn't like Brambleclaw being named after his father, some fans prefer to use the Brambleflower after his mother Goldenflower.
Runningnose has allergies. This is because he has a perpetually runny nose that he can't cure.
Speckletail's daughter Mistlekit died offscreen.
It is hinted at several times in Bluestar's Prophecy that there was some romantic interest going on between Stormtail and Dappletail, even after Moonflower's death. Awhile down the road, Dappletail births two kits that — while sources name Runningwind as the father — match Stormtail's description, alluding to a quiet relationship between the two.
Due to the inconsistencies around the idea of "fading"note a relatively minor cat like Micah makes an appearance in Squirrelflight's Hope when there is reasonably no way he should still be remembered, and the general horror at the idea, fans have come up with the idea that it's actually just a transition and cats who fade actually get reincarnated or move into a higher realm beyond what the living can interact with (but where things like the Three's powers come from).
Cats that have their cause of death listed as "painful lumps" are widely theorized to have died of cancer, such as Shellheart and Pebblefur.
Some fans believe that Mapleshade not only targeted Crookedstar because of his family lineage connecting him to her former mate Appledusk, but because he looks remarkably similar to his great-grandfather as well.
There is some speculation suggesting that Pinestar became Henry — the "fat, old tabby" Rusty and Smudge mention at the beginning of Into The Wild — after he gave up leadership of ThunderClan to become a kittypet, mostly due to physical description and Smudge mentions he's skilled enough to have caught a robin when he ventured into the woods. However, there is one hole in the theory: Pinestar was already dead by the time Bluefur became Bluestar, as he gave her one of her nine lives.
Some fanfics have Clan cats refer to female cats as "mollies" instead of "she-cats," as "molly" is a real word for a female cat.
after being struck by lightning in Lost Stars.