- Any toys that resemble real-life firearms or weapons. The crack epidemic and ensuing gang wars in the '80s and '90s involved a lot of gunplay, and city kids who brandished realistic fake weapons risked being shot for real by criminals, police officers, or both. To safeguard kids, toy guns now are brightly colored (black is disappearing), usually have a tell-all orange tip barrel, have exaggerated proportions so that they can be recognized as toys at first glance, can't be easily painted or remodeled to resemble the real thing, and represent no commonly available firearms, looking more like fantasy/Science Fiction weapons than anything. (Because of these laws, the original Transformers toy of Megatron, which transformed into a realistic handgun cannot be reissued in the US.) Cap guns, meanwhile, have vanished due to the public's tendency to mistake their noise for real gunshots. Water guns like Super Soakers, however, are still very popular, largely because nobody could confuse them for real guns. The only "toy" guns still sold today that resemble real weapons are BB rifles, which get by on the Grandfather Clause (the Daisy Red Ryder BB gun was famously featured in A Christmas Story, after all), and Airsoft guns, which are often very expensive and marketed strictly to hobbyists and the like. And even in those cases, nearly half of all US states regulate their sale and use.
- Toy soldiers based on any historical war. Back in the days of World War II, the US military were undoubtedly the rough 'n' ready good guys fighting valiantly for liberty and justice against the evil regimes of Nazi Germany and Imperial Japan. With the onset of The Vietnam War and especially The War on Terror, we began seeing more shades of gray, such as innocent people on "the other side" suffering as a result of the wars, the American government making some objectively boneheaded decisions, and parents who served in the wars either dying or coming home crippled. It became more-or-less clear that War Is Hell and not the kind of thing we want our kids glorifying.
- This can be seen easily with G.I. Joe. The original figures were explicitly designed as American infantrymen, but after Vietnam, the line shifted focus to adventure stories or battling absurd supervillains, largely jettisoning anything that could be associated with an actual war. It's hard to imagine, for instance, Snake-Eyes in Syria fighting ISIS.
- Bratz dolls. When they first came out at the Turn of the Millennium, the fashion dolls were hugely popular in a way no prior Barbie rival had ever been, and very controversial due to their "slutty" appearance for characters who were supposed to be teenagers. This controversy was compounded by Mattel's plagiarism lawsuit against Bratz dolls manufacturer MGA, with Mattel winning. Now thanks to said controversy dying out, several failed retools and spinoffs, a poorly-received live-action movie starring Jon Voight, a lengthy hiatus due to legal issues, and the rise of doll lines such as Monster High and the My Little Pony: Equestria Girls that feature Loads and Loads of Characters with truly distinctive personalities and strong World Building, the Bratz franchise is struggling to stay afloat and is virtually dead. There's a good chance most toy-buying kids aren't aware the line still exists, or even existed. A reboot of the line came along in 2015 but many, even former fans, doubt it will last long. Even Moxie Girlz, created by the same company to replace it, isn't doing too well and now exists mainly as a Lighter and Softer budget line.
- Lawn Darts have not been popular in the United States since the early 70s, and not because it was a fad that died out. Rather, they were very, very dangerous. This ban was challenged in the late 70s, and the ban was lifted with the stipulation that they not be marketed as toys, but after a fatal accident in 1987, the Consumer Product Safety commission reinstated the full ban. Since then, they've been outlawed in Canada too, and it's unlikely they'll be popular again. Some satirical modern fiction (like say, Batman Returns and Celebrity Deathmatch) have since portrayed them as toy-themed weapons. They're even on the Toys section of the So Bad, It's Horrible page.
- Sea Monkeys was one of those "hard to believe it was popular" fads. Inspired by ant farms, these were homegrown brine shrimp marketed as novelty aquarium pets, heavily marketed, especially in comic books starting in 1957. Unfortunately, the advertising was rather deceptive, showing humanoid animals that bore no real resemblance to the actual shrimp, and many buyers were disappointed by the dissimilarity and by the short lifespan of the animals, and the fad eventually died out. However, Sea Monkeys had one moment of glory in 1998, when astronaut John Glenn took some aboard Space Shuttle Discovery during mission STS-95. After nine days in space, they were returned to Earth, and hatched eight weeks later apparently unaffected by their travels.
- LJN Toys used to be up there with Mattel. However, the aforementioned "making toy guns resemble real life firearms" ended up doing them in when their Entertech line of water guns was used in actual robberies due to their likeness to real guns, while two children carrying them were shot dead by police for the same reason. Now, they are mostly remembered for two things: abysmal licensed video games, and The Angry Video Game Nerd trashing said games. The company's swan song was in 1995, when Acclaim (which itself went defunct), which acquired LJN in 1990, retired the LJN brand name. There was a short 2000 revival, but that lasted one game, and it was so poorly received that it's rumored Acclaim used the logo to protect their image.
- Among Transformers fans, frenzy_rumble was a popular customizer who made everything into a Combining Mecha. The desire to get updates of classic combiners and new ones (such as turning perennial Ensemble Darkhorses Dinobots into a combiner) got him a lot of praise. However, the fandom soon grew tired of the perceived laziness of his designs (that they were essentially Transformers bolted onto an existing skeleton, rather than turning into the limbs themselves) and rumors of stealing ideas and not following through on commissions didn't help. Finally, the third-party boom completely put him to rest. Suddenly companies making better combiners made him obsolete.
Deader Than Disco / Toys
If there's anything parents know, it's how kids obsess over the latest thing, then forget about it after a week or so, hence why the Trend Aesop exists.