Remember that bit on the main page about how the internet made fact-checking easier, but this series is loaded to the gills with deep fanon? This is because the anime first got popular in the west around the same time the internet began to have mass-appeal, but still long before the advent of things like downloadable anime/manga, wikis, of any sort of centrally-located source of information. All the information you could get was from fan-sites, and from fan-fiction. And many, many people were exposed to the series through the internet via those fanworks, who would then become fans and write their own fanfics and make their own sites, despite not having seen the series at all. Many fanfics would note that they hadn't seen the anime, but their own depictions lined up with fanfics and fansites they read and thusly "should be accurate". No main fandom hub either, so all the little factions could be happy in their own version, with cross-pollination coming from the fans that would move from site to site, spreading ideas and fics. So many of the following examples were born because half the fandom was two or three degrees removed from the original work (and had no way to get access to the originals), but were just as invested in the interpretation of it they had been exposed to, fanwank and all.
One of the most egregious examples of Fanon taking off (à la Akane turning in to a psycho-hose beast) is Ranma's refusal to fight women. He's never once said that, and in fact at least 40% of his fights are with women. The part that is true is that he has stated that he dislikes fighting women, and it's obvious that he generally holds back against them. As can be seen by the fact that his default approach to a female opponent he perceives as weaker than himself, such as Shampoo or Konatsu (initially), is generally to try and incapacitate without causing any damage, if he believes them to be sufficiently weaker to overcome in this manner. If they are stronger than himself, such as Herb, Cologne, and Rouge- Asura, or the situation is dire, such as with Kiima (who he simply opted to pin to a wall in their first less serious confrontation, but then buried in a rockslide in a more serious duel), he is perfectly willing to attack with full force, regardless if he is in male or female form. Likewise, if his initial assessment of a female opponent's weakness is proven wrong, he immediately ups the ante, as shown in Konatsu's introductory story arc.
One of the reasons for this belief among fans may be that Ranma explicitly states that he doesn't fight girls in the English dub of the anime, the first time he and Akane attend Furinkan High School together. Translations of the original Japanese, however, seem to suggest that Ranma is actually only stating his belief that he could never fight a woman and use his full strength. A subtle difference, and we are dealing with translation here, so degrees of interpretation are understandable.
Building on from the above fanon about Ranma's refusal to fight women, and fueled by Ranma's tendency to be be an unthinking Smug Super when sparring with Akane, a very common fanon is that Ranma actively believes that women are weak and incompetent. Such misogynous statements are never uttered in either manga or anime canon, and probably have more to do with the Values Dissonance of late 80s Japanese comedy manga vs. more modern Western sentiments — basically, if Ranma actually did fight women casually, he'd look like an abusive jerk.
Some fanfic authors tend to think that Ranma barely knows how to use weapons and is completely dismissive of anyone who would use them, whereas in the actual canon Ranma is repeatedly explicitly shown as a highly skilled staff fighter, which he can extend to spears and bokken. He's also skilled with nunchaku, knives, and several kinds of swords (Genma himself performs very impressive sword techniques very offhandedly.) He also regularly makes use of anything within reach as a non-lethal Improvised Weapon, including stones and even using a pencil with amazing aim.
Some fans believe that Ranma does not show a strong reverence for life, to the point of being dismissive if he kills someone. However, this is inaccurate, as he tends to avoid killing combat even when an opponent more dangerous than himself is attempting to kill him, and saves any lives he notices in danger, including said people who just attempted to kill him and could pose a future danger, such as Herb, or people he genuinely despises, such as Happosai and Nabiki, even taking damage for their sake. On the other hand, when outclassed by the reprehensible Saffron, with Akane's life on the line, he stated that he would make a specific exception. He is obviously also not worried about knocking out superhuman opponents by burying them in rock slides of ambushing them with blunt objects from behind.
In contrast, another, much more popular fanon is that Ranma absolutely hates fighting to kill and is deeply scarred by killing Saffron. Whilst "Ranma doesn't like killing" is a very logical deduction and probably more or less true, given Ranma's avoidance of lethal force in the vast majority of canon, he shows neither hesitation in upgrading to lethal force against Saffron, nor any particular qualms about having killed Saffron (who admittedly came right back to life as a baby afterwards).
Despite what is often shown in fanfics, Ranma actually does not use the Moko Takabisha technique that often in the series. In fact he barely uses it at all after his fight with Ryoga. In fact, the next time it came up in the manga was five volumes later, in Miss Hinako's introductory story: when he used a "Double Moko Takabisha" attack against her, there was a sidebar explaining the basic technique, indicating how uncommon it is.
Ranma does not stutter whenever he tries to say the word 'cat', nor does he fly into a panic whenever someone else mentions cats. The only time he shows this fear is when a cat is actually present.
A minor bit of fanon frequently shows up in fanfics that give Ranma an extended hospital visit for surgery or other serious treatment — that doctors, once aware of his curse, will always insist Ranma remain in female form because it's harder to trigger a change and thus cause unexpected further damage. The few times we actually see Ranma in a hospital in the source material, he's always in male form.
An overlap of fanon and Creator Provincialism inevitably shows up with "Ranma gets pregnant" fics. It's always justified that Ranma chooses to undergo what is canonically one of his worst nightmares because Good Girls Avoid Abortion; this is because most fanfic authors are Westerners and abortion is an extremely controversial and polarizing subject in the West. In Japan, however, abortion is far less controversial - it has been legal and culturaly acceptable for centuries - and it's actually the preferred way of handling family size control, as opposed to the far more legally and culturally restricted use of birth control. So, Ranma would not believe that Good Girls Avoid Abortion because that trope is not a part of Japanese culture.
Many fans assert that Ranma objects to Love Potions and other forms of Mind Control on a moral level. In the manga, this isn't true at all. As a matter of fact, in Akari's first appearance, Ranma's reaction to learning of her feelings for Ryoga was to try and acquire an incense that created a similar suggestion effect as the Hypnotic Mushrooms, and use it to make Ryoga hug whoever said the word "pig", thus breaking the ice between him and Akari. Likewise, in the Pink and Link storyline, upon evading an attack by the Single-Minded Twins with a magical flower that conditions whoever is wearing it into a "perfect housewife" type, Ranma's immediate response is to try and apply one to Akane, who promptly beats him up for doing so. Later on, during the Mt. Phoenix arc, Ranma tried to have both Genma and Shampoo fall under his command when they were accidentally trapped by the Imprinting Eggs, only to have someone else hijack his attempts.
Ranma's personal promise to himself that he'll keep Ryoga's curse a secret is often milked for drama or angst. The truth is, once Ranma figures out that Ryoga intends to milk the "P-chan" thing for all it's worth, and will fight Ranma to keep his place in Akane's bed, he loses all interest in upholding it. The only person Ranma won't tell is Akane, because once she figures out he was complicit in the deception, she'll go absolutely ballistic at him. It doesn't hurt that Ranma can use Ryoga's fear of being revealed to mess with Ryoga.
A common Fanon idea is that Ranma is not only a good martial artists, but also the best in the world, with him defeating Saffron being a major show of power that cows even Cologne. In fact, while Ranma is clearly talented, he's still not the absolute best. Herb surpasses him in strength and ability even after his one-time victory, and the same can be said of Happosai and Cologne. Furthermore, Ranma's defeat of Saffron is owed more to Ranma getting his hand on a magic item that countered Saffron and help from Akane than to any of Ranma's own direct skills.
Fanon often portrays as Ranma as extremely bluntly spoken, deliberately rude, and thoughtlessly offensive. That's...not entirely fanon, but greatly simplified and over-exaggerated by fanfiction. In canon, Ranma is quite capable of being polite and modest, though he does have a brash side and flares up quickly when he feels insulted.
Similarly to "Ranma is a rude and arrogant jerk", fanon often portrays Ranma as exclusively starting the arguments between himself and Akane. In actual canon, Akane tends to start the fights just as often as he does, and many times Ranma is rude to her because of something abrasive she said first.
Plenty of fanfics portray Ranma as being disdainful of "shortcuts" to power, seeing those who use them rather than earning their strength through hard work as cheats. This fanon is often used in reference to the two arcs where Akane gets a power up. (Super Soba and Battle Dougi arcs). In truth, Ranma does not have any particular problem with shortcuts or tricks to get power. He's never shown bothered by or judging Ryoga for getting the Mark of the Gods, and he fought just as hard as everyone else for the "Noodle of Power" that they believed would give a person the strength of 100 men. In fact, a major part of the conflict in the Battle Dougi arc is that Ranma is upset that he didn't get the powerup, and even tries to force it on himself at one point.
Akane's frustration with Ranma "not fighting back" or "not taking her seriously as a fighter" has only been mentioned a handful of times in the anime and manga. In fanon, it tends to be brought up much more frequently. Furthermore, the canon depiction of these events tends to be quite different to the "I'm a martial artist too and can do anything as well as you can" approach presented in fanon. In the source material, at the very least manga-version Akane knows her limitations full well, Ranma and Shampoo certainly tend to rub them in her face, and she is shown as extremely frustrated about her lack of success in this and various other areas. She has been shown as angered that Ranma won't train her, and "take her seriously" in that manner, but has no illusions whatsoever about her relative skill, and is trying extremely hard with everything, usually without any results, hence the frustration and need of guidance. However, she is shown to increase her samurai weaponry skills to Kuno's regular level, does turn more acrobatic, and in the anime at least grows considerably when directed by Ryoga. However, the regular dynamic of Akane being the Butt-Monkey for the umpteenth time has twice been spoofed in 'karmic payback' situations when she has been given a temporary boost that allows her to recurrently effortlessly defeat Ranma whenever he challenges her.
Building upon the above, another fanon myth is that Akane does not try to get better or only relies on items, and that Ranma only gets upset about being shown up if it is done in this unfair manner. The reality is that Akane actively trains, if not very often on-screen, and most of her "power-ups" have been accidentalnote eating Happosai's Super Strength-bestowing soba noodles without knowing what they were, getting enchanted costumes from other people without knowing their magical nature. This fanon is probably cemented by just how possessive Akane can get of her powerups when she gets them, though that's at least partially because of how quickly Ranma sinks into jealous Jerkass mode as a result.
The idea that Akane is a horrible martial artist, as said by every character ever in a non-Akane fanfic. This is an unfortunate result of how much the Overshadowed by Awesome trope is made use of in the canon; people pay attention to Akane's moments of being the Damsel in Distress and how Ranma gets to do all the "real fighting" and super-cool techniques, and so they ignore Akane's efforts by comparison even as she pulls off superhuman feats of skill, strength, or agility of her own.
Akane's "terrible" skill gets parodied in this fanfic, with the author noting that while Akane sucks compared to Ranma, she only sucks in comparison and "real" people would be a total joke to her.
One semi-fanonical aspect is Akane's use of a hammer to hit Ranma when she gets angry. She has done this, but whenever she does, it's almost exclusively in the manga; in the anime, she is more likely to use a shinai (a kendo training blade made of bundled bamboo sticks on a sword hilt) than anything else. Really, the only time Akane has been seen using a hammer in anger in the anime is during the first movie. Fanon also overstates how likely she is to use a weapon at all, let alone a mallet; Akane typically punches or kicks him, or grabs a nearby heavy object (like a desk, a table, a stone lantern, or whatever was already in her hand if it's sturdy enough).
The case of Tatewaki Kuno and his Dramatic Thunder is an unusual one. It does happen on several occasions in both manga and anime, especially if there's already a storm going, but it's not a prominent Running Gag. In contrast, many fanfics claim that this happens every time Kuno feels like dramatically calling himself "Blue Thunder", or even say it's a technique he learned to look cool.
One thing that commonly pops up in Ranma fanfiction is the idea of Kuno calling Ranma a "vile sorcerer". In canon, Kuno has never called Ranma this. He does believe in magic (ironically), and also believes Ranma practices magic, for more irony, but he doesn't actually call Ranma a sorcerer. His usual address for Ranma is as "Saotome", with Japanese Honorifics emphasizing the fact that Ranma is, technically, Kuno's social inferior.
Another bit of Kuno fanon is that he is in complete denial of his vast losing streak against Ranma. This is inaccurate; the reality is that Kuno is defiant of his many losses, not ignorant — he accepts that he has been beaten, but refuses to stop trying, confident in an ultimate victory.
Fanon to the contrary, Kuno doesn't claim that Ranma beats him through the use of black magic. He does accept that Ranma is skilled, but of course he doesn't say as such for his own pride.
Many fanfics claim that Kuno and Nabiki are a former couple, who broke up over Kuno's infatuation for Nabiki's little sister. The reality is that no such past relationship is ever mentioned in the canon, and the one "hint" of it is the brief sequence of Nabiki tending to Kuno in the sickbed after Akane clobbers him for morning, during the first five chapters/episodes of the series. Whilst the two express mutual disgust at the idea of dating each other, they speak of it in the present tense, never in any way that implies they used to date before.
According to most fanfiction out there, Ryoga only ever makes his entrances by jumping out of nowhere to try to kill Ranma, usually calling out "Ranma, prepare to die!" Even though Ryoga is, as far as pure fighting is concerned, one of the most honorable combatants in the series, and he only pulled the above twice in the series —his introduction, and at the start of the Bakusai Tenketsu arc. For all the series that came afterwards, if he's looking for a fight, as in, an actual duel, he always sends formal letters (or postcards) of challenge citing Ranma at a specific date and place. If he's simply angry about something (he thinks) Ranma did (to himself or Akane) and wants to vent, he also makes his presence known and THEN attacks, he never ambushes Ranma if he can help it. Ironically, it's Ranma who tends to show up out of nowhere to annoy Ryoga while the latter is distracted, either monologuing or deep in thought.
The Shi Shi Hokodan uses the Battle Aura generated by a depressed, miserable person to unleash a crushing blast of spiritual force. At one point in the battle, Cologne call the technique a terrible one and that it's foolish to fight with it, likening it to two lions clawing at each other even as they tumble down a cliff. Though this is clarified to mean it's pointless to duel with the Shi Shi Hokodan (as the worse you're doing, the more powerful it is — but the better you're doing, the less powerful it is, so the fighters just end up seesawing back and forth until they pummel each other senseless), many fans adopted the stance that the Shi Shi Hokodan actually induces despair, causing a martial artist who learned and uses it to become more and more depressed and melancholy, or that the technique requires a despair-filled life at all times. This despite the fact Ryoga went out of his way to make himself as miserable as possible for the fight, so he could stay "focused" and beat Ranma no matter how well the fight went, his personality was unchanged in subsequent stories and Ryoga was, frankly, The Eeyore of the series anyway.
A very common bit of fanon is that Ryoga flatly outclasses Ranma in terms of their respective levels of Super Strength. In actual canon... we don't actually know how they stack up! Ryoga has pulled off many insane feats, but so has Ranma. In fact, Ranma has on several occasions shown signs of matching Ryoga's strength despite being in his female form, which is known to have less raw strength than his male form. For example, Ranma-chan has been shown wielding Ryoga's heavy umbrella with equal ease as Ryoga during their first formal duel in the series, something lampshaded in the manga version of the story. Likewise, in the manga version of the Martial Arts Skating storyline, Ranma-chan is likewise shown being able to keep afloat despite Ryoga and Azusa not only sitting on her head, but Ryoga also lifting an enormous mass of ice at the same time. Whilst Ryoga was able to throw Ranma-chan around like a ragdoll in the manga version of the Checkers storyline, or bearhug her so hard she vomited up the waterproof soap she had accidentally swallowed, that's more of a Strength vs. Toughness affair. The two have literally never gone in a direct strength-to-strength competition in the entire series, and no Word of God as to who is physically stronger exists. The fanon mostly exists because of Ryoga's tendency to smash everything around him when distracted.
Chinese Amazon Fanon
In fanfiction, whenever Shampoo's favored weapons are mentioned, they are almost invariably named "bonbori", which is actually a kind of spherical lantern hung outdoors. In fact, it's only recently that they've begun to use the real name; chúi, an obscure form of Chinese mace, noted for being exceptionally heavy (those oversized heads are traditionally completely solid balls of steel).
There is little evidence that Shampoo's village is especially hostile to men and absolutely none that suggests that the local men are enslaved. All we know is that Shampoo dislikes Mousse (in much the same way Ukyo dislikes Tsubasa, or Akane dislikes Kuno,) that the village is particularly renowned for its fighting women, and that any warrior woman of the village is required to marry the outsider male who defeats her, saying nothing about local men one way or another. You could just as easily say that the village's laws are misogynist, and it hardly seems likely that Cologne would waste her time teaching advanced martial art techniques to a 'slave'. Amusingly, one (non-canon) PC Engine fighting game used Shampoo's father as a final boss, appearing after Cologne, and he was the most formidableopponent in the game.
The idea that Cologne is some kind of high-ranked authority figure in her home village, which may or may not be portrayed as a Lady Land. This seems to stem entirely from the fact that her introductory anime episode was giving the title "This Old Gal's The Leader of the Amazon Tribe?". It's not likely in canon, given how she moved to Nerima without the slightest fuss and would have no problems staying there permanently, given how Shampoo's lie that she and Ranma are married and have settled in Nerima is swallowed so easily by the newspaper back in their village.
Which also brings up the idea that Cologne and Shampoo expect to take Ranma back to China once he weds Shampoo. Canonically, all they ever insist is that Ranma marry Shampoo; nothing about coming back to China is ever stated. Both the anime ("Here Comes Ranma's Mom!") and the manga (Pink and Link saga) actually heavily imply Shampoo wouldn't think twice about staying in Japan permanently if she does manage to catch Ranma. In the anime version, thinking Ranma is moving in with his mother, she comes running up with all her worldly goods and begs him to take her with him, as Cologne has okayed it. In the manga, she's quite content with lying to her village newspaper that she and Ranma are already married, they've merely chosen to stay in Japan.
Related to this is the idea that the kiss Shampoo gave to Ranma after he beat her as a male is a formalized "Kiss of Marriage" existing as a counterpart to the "Kiss of Death" Shampoo gave Ranma when a girl. In truth, this phrase is never used in the story.
Although it arguably originated as a joke in an early fic, by the late 1990's it became an automatic assumption that every Amazon has a name based on a toiletry item.
Gregg "Metroanime" Sharp originated a score or more named and fleshed-out supporting Amazon characters as part of his "Bet" multiverse. By the late 1990's-early 2000's they had begun to leak out into other writers' works.
No, Shampoo's Chinese name isn't "Xianpu", "Xian Pu", "Xian Fu", or whatever other "Xian X" namenote It's Shānpú.. Neither Mousse's original name "Mu Tse", "Mu Tsu", "Mu Zi", or "Mu Zu"note It's Mùsī. Similar widespread mistakes abound regarding most if not all of the other Chinese characters' Chinese names.
Speaking of Mousse, he's almost always written as the fastest of the "big three" (Ranma, Ryoga and Mousse) in fanfic, probably to balance Ryoga's Canon portrayal as the strongest of them (or at least toughest, see above) and put Ranma in the middle with both strength and speed. Possibly inspired by the fact that in their battles with Herb, he was paired up with Mint, who was the speediest of the three Musk warriors.
Many fanfics portray the use of Love Potions as Shampoo's go-to schtick, which is something of an exaggeration. Although it's true that the anime version of Shampoo is more of a perpetrator in her Love Potion stories, Shampoo has still only been involved in five of the nine stories note the Hypnotic Mushrooms, Love Potion bracelet and Reversal Jewel storylines, the anime's Red String of Fate, Love Potion bandage and Mushrooms of Love stories, the manga's Together Umbrella, Fishing Pole of Love and Akari Unryu Appears stories with that theme between the two continuities. Of these stories, only three appear in the manga, Shampoo was an opportunist taking advantage of the Hypnotic Mushrooms in the manga, got involved by chance in the Love Potion Bandaid story, and was an unwitting victim in the Reversal Jewel storyline. Only the Love Potion bracelet and Red String of Fate stories were Shampoo deliberately using love potions after finding them.
Similarly to Akane and her use of a mallet, the idea that Shampoo always encounters Ranma by running him over on her bicycle is both overstated and a manga-exclusive event; its only animated appearance was in the Christmas OAV.
Many assume that Cologne intentionally threw Shampoo into the Spring of Drowned Cat as punishment for not marrying Ranma, and that she's forbidden from seeking the cure until she successfully marry's him. For the first part, her punishment was to begin her training all over again, and that included a fight at the Jusenkyo, where she did fall into the spring. For the second part, neither character has ever mentioned being forbidden from seeking a cure. Much like with the Musk/Joketsuzoku conflict above, it's a logical inferral, given that Shampoo never gets a cure in the series despite its hindrance to her goals and the way that both Shampoo and Cologne blame Ranma for her being cursed, but it's never directly stated that she can't get cured.
Weirdly, in an anime season 7 filler episode where the Jusenkyo Guide comes to Japan, Cologne explicitly suggests that they could use this as an opportunity to cure Shampoo.
In fanfiction, Shampoo is often portrayed as The Nicknamer, with a tendency to refer to the other members of the cast by various nicknames; the most ubiquitous are "Violent Girl" and "Kitchen Destroyer" for Akane; "Spatula Girl" for Ukyo; "Mercenary Girl" for Nabiki; "Nice Girl" for Kasumi; "Stupid/Old Panda" for Genma; and "Lost Boy" for Ryoga. Whilst not a far leap from her canonical Hulk Speak or Asian Speekee Engrish tendencies, she never actually engages in this kind of behavior in the canon.
Many fanfics assert that Ukyo is Ranma's "Best Friend", on the grounds that they were close as children, Ranma is clearly happy to see her again (though he was the same way when he recognized Ryoga the first time) and they don't try to brutalize each other the way Ranma and Ryoga do. As part of this, they usually assert that Ranma feels closer to her and goes to her for an understanding ear, which is a falsehood; Ranma is an intensely private individual and never talks to anyone about anything, to the point that Ukyo and Shampoo's existence came as a complete surprise to each other in the manga.
Some fanfics like to assert that Ukyo is actually Ranma's "first fiancee", which is based on the technicality that while the Tendo engagement was arranged first, Ukyo was the first of Ranma's canonical love interests that he actually met face-to-face. Which is rather undercut by the fact that he didn't realise she was a girl at the time.
It's common for fanfics to blame Ukyo's vendetta-quest on her father, whereas the canon quite blatantly states it was all Ukyo's idea, with no mention of her father being made after she notes how Genma swindled him. This may stem from the fact that Ukyo is fundamentally a direct recycling of Ryuunosuke Fujinami from Urusei Yatsura, a Wholesome Crossdresserbifauxnen who was forced into her gender-queer status by an absolute nut of a father, leading to early fans of both shows confusing the two characters and this getting propagated by fanfiction.
Another bit of Fanon related to Ukyo is that many fanfics assert that "She's known Ranma since he was very little." This is incorrect at best, considering that they were together as kids only for a very short time, and, although he does remember her when she shows up again (and he's thrilled to see Ol' Pal Ukyo again) she was a boy in those memories and he didn't think anywhere near as much of her as she thought of him. This bit of fanon is especially jarring when bookended by accusations of abandonment that is far more accurate. Thus, it would be more precise to say that "she knew Ranma when he was little."
Genma & Nodoka Fanon
The belief that the Saotome home, in which Nodoka lives alone, is located in Azabu-Juuban. This is especially prevalent in Fuku Fics written before the Ranma manga was fully published in the West.
To hear it from some fans, Genma made dozens and dozens of Childhood Marriage Promises for Ranma in exchange for food. He only did that twice in the manga (although the anime also introduced Kaori Daitokuji, and a single other claimant, in a filler episode). Chronologically, the first time it was together with Soun (see below), and the second time it was regarding Ukyo. In Ukyo's case, it was her father who made the initial offer and Genma asked Ranma (at least so he claims) what he thought (and then stole the food cart anyway, but that's another issue). The reason this fanon is so popular is because... well, Genma's a scheming scoundrel in canon, and if he canonically did it twice (four times in the anime), it's reasonable to suppose he's done it other times as well.
Conveniently, fanonalways forgets that not only Genma, but Soun also, paid off their combined meal at the Chardin restaurant with an as of yet unborn and unspecified daughter, in a much less dire situation than Genma's promise to Mr. Daitokuji. That said, this case is excused, or at least isn't viewed in the same light for several reasons: Soun is usually a much more moral character, fans feel less animosity towards him, and since Genma didn't actually have any daughters it fell on Soun to pay up... at which point Ranma volunteered to take up the promise and thus relieved him of the burden.
Contrary to what many fics claim, Genma is not fat. It's probably bleed-over from his panda form (which is actually referred to by a female panda spirit at one point as being fat and ugly), plus the shapeless gi he wears, but a lot of fanfic writers tend to depict him as a blubbery mass of lard. While it's true that he's not slender (or as good looking as most of the rest of the regular cast), every time he's shown without clothes in the series, his build seems to be fairly flat-bellied for all his stockiness — in fact, the first time he's shown without his shirt on in the anime, he's shown with a six-pack. Its especially egregious in fics where Ranma is shown gagging at seeing Genma's allegedly bloated body nude when he is shown regularly bathing with his father all throughout the manga and anime.
An extremely common portrayal of Nodoka in fanfiction is as somebody desperately waiting for Ranma to make her a grandmother, to the point of her pulling out fans and doing a happy dance when she thinks on the subject. This is often used in fics either to make her suggest that Ranma should Marry Them All (whether he does or not depends on the fic) or to have her support Ranma abandoning his engagement with Akane Tendo for a more openly affectionate (and usually crossover-based) girl. In canon, Nodoka has never expressed any opinions on or interest in grandchildren at all; her sole concern is always whether or not Ranma is acting "manly".
As for the sub-fanon of Nodoka supporting Ranma having as many girlfriends as possible, that also is never mentioned in canon. Indeed, it's possible to argue whether or not she's even aware of Ranma's love interests outside of his engagement to Akane.
A common bit of fanon about Genma is that he never uses Ranma's name, always calling him "boy" or "the boy". Whilst he has addressed Ranma by the term on occasion, the canon is that Genma frequently uses Ranma's name, whether addressing Ranma himself or talking to others about him.
Post-manga fanfics often mention Nodoka using her sword to intimidate Ranma into doing whatever she wishes, either for comedy or plot convenience. This is... not without its roots in canon, but never actually seen in canon — mainly because Nodoka's joining the Tendo Dojo was the last story before the Phoenix People arc. But, during the "Ranma moves out" story that takes place between Ranma's curse being revealed to Nodoka and the Phoenix People story, there is a scene where Ranma is shown sulking over Akane's apparent disinterest in their separation. In response, Nodoka is shown clutching her sword intently and commenting "Dear... Ranma isn't acting very manly..." whilst a visibly nervous Genma physically bars her from Ranma's room, holding up a sign with "Easy, easy" written on it. So, this kind of behavior, whilst technically fanonical, is not a far logical stretch from canon.
The name "Kimiko Tendo" tends to pop up with frightening regularity when referring to Soun Tendo's late wife, even though it originated in Fan Fic — canonically, we have no idea what her first name was.
Kasumi Tendo has quite a bit of fanon built up around her as well, which makes sense given she's one of the main characters we know the least about:
Firstly, there's the topic of her libido. Many view her as the most prudish or sexually naive member of the Tendo family, to the point that there's at least one fanfic portraying her as a full-fledged asexual. Other fics like to portray her as secretly having an incredibly powerful (if repressed) libido - whilst this began as a jokenote for the combination of the inherent comedy of the Covert Pervert trope and the irony that the sister that notoriously repressed Akane looks up to is actually a huge pervert that was particularly handy for erotic fiction (limes and lemons), it's old enough that theoretically there are fans who actually take it seriously. The truth is that we just don't know how Kasumi feels about sex. She does have at least some very "proper" views on the matter, in that she is seen declaring that sex shouldn't happen until after marriage during the first P-chan story, but beyond that it's a mystery. In fact, in the anime continuity, a Beach Episode that occured in the final season features Kasumi wearing the second smallest bikini (after Kodachi's dental-floss-and-postage-stamp number) of the entire female cast.
Kasumi's never-fading positive attitude despite the mayhem surrounding her has led to many fans trying to figure out why. This has resulted in portrayals of Kasumi as everything from a Stepford Smiler to being so out-of-touch that she legitimately doesn't understand what's going on. A rarer take invoked in some dark/Tendo-bashing fics involves portraying her as a hidden sadist, who secretly enjoys all the chaos because it's fun for her to watch, although even now few people actually take that portrayal seriously. In canon, Kasumi is portrayed as just more of a very polite optimist, who has gone so far as to scold people for causing trouble... which admittedly grew less common as she got an increasingly worse case of flanderization.
Most fans take it as canon that Kasumi either is in love with Dr Tofu, that she is only attracted to older men, or both. The "older men" thing does have its roots in canon, as the very first story has Kasumi hoping that Ranma will be older than her — but we don't know why she wishes this, whether it be a fetish or just a belief that only older men are mature and responsible. As for the Dr. Tofu thing... canonically, she's completely unaware that he feels anything for her, due to his switching into bungling idiot mode whenever she's nearby.
A common, if older, fanon trope is that Nabiki's High School Hustler scams are actually what is supporting the Tendo household. This completely changes her personality from one of the only characters explicitly said to be evil to a noble and self-sacrificing character. This likely came about due a mixture of however Soun makes money not being important to the plot and some earlier fanfic writers not realizing that even several thousand yen is not much money in the grand scheme of things via Cross Cultural Kerfuffle.
When it's brought up at all, the typical idea in fanfics is that Pantyhose Taro augmented his "winged minotaur" form with Combat Tentacles by applying Spring Of Drowned Octopus water "to eight specific points on his back", probably because, when splashed with Spring of Drowned Twins water, Happosai grew an extra bump on the head, since only his first bump got wet. This, despite him also being able to shoot ink from his fingertips. Canonically, we don't know what he did to gain those partial octopus traits beyond "he used the Octopus Spring Water" (as explicitly stated by the Guide over the phone), and any kind of theorizing is just that, theorizing.
Jusenkyo curses are often ascribed the trait of being Laser-Guided Karma or a punishing form of Personality Powers in fanfiction. Sometimes even both. Ranma is often said to have "earned" his Gender Bender status by virtue of being a He-Man Woman Hater type misogynist — which ties into the aforementioned fanon about him not hitting girls — with these fics usually adding that he will only gain some form of control/freedom from the curse when he realizes that girls aren't weak. Genma's panda form, meanwhile, is commonly suggested as being a sign of what a lazy, gluttonous individual he is, and a poke at his (admittedlyjustifiable) reputation as being a terrible father and husband.
A belief that still persists to this day is that Jusenkyo is destroyed at the end of the penultimate storyline, trapping Ranma and the others with their curses forever. This probably stems from the earliest access to that storyline being mistranslations uploaded to the internet, which are incorrect. Jusenkyo does get flooded by the end of that story, but it's made clear that it will eventually go back to normal; it's just dangerous to hang around whilst all the springs are mixing together. In fact, there's a single-panel shot of the Jusenkyo guide ringing the Tendo Dojo in the next story to essentially say "Flooding has gone down, so all springs are back to normal; here's a cask of Drowned Man Spring water for you as thanks for saving my daughter". The arrival of this cask promptly sets off the Disaster Dominoes.
Saffron, the Phoenix King, is not a god, a demigod, or any other such thing, and never claimed to be one, even if his Supernatural Martial Arts powers were the strongest in the manga. Note that this doesn't stop Fan Fic writers from claiming that Ranma "has killed a god", and then have the cast reacting with appropriate awe.
Also, many fanfics gloss over the fact that Saffron came back to life as an infant afterwards, although admittedly that doesn't change the fact Ranma did manage to kill him in the first place.
There's a whole "hierarchy of bust sizes" many fans subscribe to that puts Shampoo or Ranma at the top (oddly enough, in lemon Kasumi often seems to get top billing) and Akane and Ukyo at the bottom. Actually, as far as manga canon goes the ONLY thing we know about relative sizes is that Ranma is bigger than Akane, and both grew up over the series. The artwork varies a bit, but none of the girls are noticeably flat nor overly busty, so it's really impossible to assume ANYTHING beyond that. The likely reason for this fanon taking off is that big breasts are seen as sexier, and both girl-Ranma and Shampoo are used for regular fanservice with their tendency to end up partially or wholly nude, in addition to Shampoo being the "exotic beauty" of the love interests. In comparison, and adding fuel to the fire, Akane and Ukyo aren't used as fanservice in the same way, are rarely seen nude (although Akane is depicted that way more than Ukyo), Akane is supposed to be a "girl next door" type (a character angle not usually portrayed as very well-endowed), and Ukyo's bifauxnen schtick naturally suggests that she's quite flatchested. It doesn't help that a prominent Japanese fanartist with a style similar to the original anime created a comparison chart and Western fans continue to pass it around as official information.
Less common than some of the fanon on this page, but, it's fairly common for older stories to portray the Musk Dynasty and the Joketsuzoku as having a bitter animosity and regularly fighting each other. Although this does actually make sense, given the Chinese Amazons are, well, a tribe of female warriors and the Musk Dynasty are a tribe of He Man Woman Haters who live in a No Woman's Land, even after giving up their Bestiality Is Depraved based Breeding Cult traditions, it's actually contradicted by canon: not only does Herb feel visiting Cologne is a viable option, she is diplomatic and helpful towards him in turn. While this doesn't necessarily remove the idea that the two tribes have fought, the fact Cologne mentions she thought the Musk Dynasty had all died out makes it obvious that there hasn't been any interaction between the tribes in a long time.
Similarly, although the arrogant xenophobia of the Phoenix People makes hostilities between them and the Joketsuzoku and/or Musk seem logical, their isolationist ways makes it very obvious in canon that they haven't been seen by Shampoo's people in a long time.