The more astute readers among you may have noticed that I haven't yet gone so far as to give anything an actual "F." That's not out of any kind-heartedness on my part, it's just that every time I got ready to give one out, I would ask myself, "is it really that bad, compared to the verminous, soul-tainting badness of Scrappy-Doo?"
Popular characters have fandoms. But sometimes, there is a character who has a hatedom. We'll call this guy The Scrappy, or more formally the Fan Unfavorite. This is not to be confused with the Hate Sink (which is a character the audience simply wishes to root against), though it can overlap if poorly written.
While much usually depends on execution, certain kinds of characters have a much higher probability of being hated:
Someone who likes another character who is either a disliked character or just shows that he/she doesn't care for the former, will often be lauded with ridicules and dislikes, with some camp calling the first character stupid, or even consider them as another Scrappy.
Note that in order for a character to qualify, the audience have to feel that the character detracts from the work. If they simply hate the character, then chances are this character is a Hate Sink, though there's nothing stopping a Hate Sink from becoming a Scrappy if they're written badly.
In addition, if the writers begin relying too heavily on an already-present character with some potentially annoying quirks (like The Unintelligible or the Non-Human Sidekick), the viewers may begin to find him/her very annoying indeed. Sometimes even a very popular character is at risk of becoming a Scrappy when he or she wears out his or her welcome by being overexposed.
Occasionally, a character intended as a Butt Monkey will backfire and become a Scrappy, too. This usually happens when a writer attempts to make his mishaps even funnier by making him an unlikeable person to whom it's very satisfying to see horrible things befall. A great idea, but sometimes it backfires when the Butt Monkey becomes so annoying that whatever happens to him in the story, audiences are haunted by the feeling that it will never be enough. An especial risk when the Butt Monkey gags are prolonged, after which even a character who's not inherently unlikeable can stop being funny and become a Damsel Scrappy. It also comes full-circle in a way; note also that many Scrappies can easily become Butt Monkeys when touched upon in fan discussions and literary parodies. In any given Hate Fic, and even in the occasional straight-up Fanfic, the author may very likely go a step further than making The Scrappy into the Butt Monkey, killing him or her off (often brutally), or giving a "hero" Scrappy a Face Heel Turn so he becomes the villain; after which he again will likely be killed off.
Strangely, at the other end of the spectrum lies another common scrappy, the Author Avatar. Unlike a Butt Monkey, they lack any sort of shame on principle; instead serving as the voice of reason, or whatever contrived argument the writer thinks is reason. If bad things happen to this character, it will be to prove that everyone else is allegedly stupider.
Some people conflate Hate Sink characters with The Scrappy, on the fact that both types are hated by fans; but the difference is that in a high percentage of cases the Hate Sink is intentionally written that way -- usually to give fans a character on whom project their irritation if the nominal villain is inactive or relateable. As the trope namer proves, a Scrappy could have been written with the initial intention of being relatable and sympathetic (and maybe was, at the beginning), but their gimmick overstayed its welcome and the fans become increasingly irritated by its existence instead.
Just as there are several types of characters that can become Scrappies, the resulting Scrappies can take many forms:
The Ethnic Scrappy, a particularly awful manifestation when the racial caricature of the Funny Foreigner becomes offensive. Ironically, this frequently results from well-intentioned attempts to make the cast of characters more diverse.
The Alien Scrappy is an Ethnic Scrappy who is a non-human sentient being. His species doesn't exist in the real world and he may be the only example of them we ever see, but he somehow still manages to come across as a distressing caricature that is insulting to them.
A Periphery Hatedom can spring up if the character is liked by the target audience but considered the Scrappy by people outside that target.
On the other hand, if the writers take a shine to The Scrappy and add some elements of Mary Sue, or otherwise put more focus on them over more popular/liked characters, this will add gasoline to the flames - they've just turned The Scrappy into the dreaded Creator's Pet. Expect anti-websites (and, in the social media era, dozens of Orkut communities/Tumblr blogs/Twitter accounts against the character); message threads that degenerate into flamefests; and a hard fast descent into loonydom.
Ironically, despite how much Scrappy has been loathed and how hard the franchise has distanced themselves from the character, at the time of his introduction, Scrappy was extremely popular. The show was already on the verge of cancellation due to low ratings, leading the writers to create the character in a last-ditch attempt to save it. Kids loved him and the ratings sky-rocketed, leading to the various format changes that happened later to give more prominence to Scrappy.
The 2002 movie, meanwhile, parodies this nicely: the gang are revealed to have kicked Scrappy out several years beforehand for attempting to take over Mystery Inc, going as far as leaving him in the middle of a deserted road (in the same box Scooby found him!) while they drive off. He then turns out to be the movie's Big Bad, becomes a hulking monstrosity bent on destroying Scooby and tries to kill the main characters, even going so far as to suck the souls out of Velma, Fred and Daphne. Following his defeat, Scooby PUNCHES HIM. Is anyone else seeing a massive subtext about Scrappy's negative influence on the show here?
It's gotten so bad that Scrappy is all but edited out of any series or movies he appeared in when they're released on DVD. Sure, the content itself is unchanged, but the trailers, cover art, and descriptions on DVD covers for Richie Rich/Scooby-Doo, Scooby and the Ghoul School, and 13 Ghosts of Scooby-Doo reduce Scrappy's presence to a bare minimum if it doesn't make it nonexistent. Cartoon Network's official website makes reference to the character's hatedom with the Shockwave game Scrappy Stinks. It's one of the oldest games on the site, having been up since at least 2000.
This is a very subjective trope, more based on the fandom than the character itself. The visceral response to The Scrappy can baffle other fans who don't take the character as seriously, or even sympathize with him or her. Often it's subjected to Double Standards particularly against female characters, somehow negative traits are often seen as less forgivable in a female character than a male character. Please only post examples of the fandom hating a character; posting Personal Scrappies could get messy, especially if the Scrappy is in at the center of a Fandom Rivalry. Die for Our Ship entries should also be placed on that trope page only, though someone could be a Scrappy because of that reason.
Should any Troper have more interest in researching Scrappies, there is in fact, an entire Wiki for Scrappies, the Loathsome Characters Wiki, where each page gives an analysis of just why the character in question deserves this Trope as a label.