"In the Time of Dinosaurs" describes Spinosaurus as having shorter hindlegs than a Tyrannosaurus, before discoveries in 2014 suggested this may have been true. However, it's also portrayed as a land-dwelling "carnosaur", so it sort of cancels out.
The idea that the Animorphs would always morph into an animal of the age of the acquired one was never explained, and used as a plot device so that they could look exactly like controllers who they acquired and impersonated. However, years after the books were published, the discovery of epigenetics, the way organisms' DNA changes while they are still alive, show that the DNA reflecting how old an acquired animal is was very accurate.
Approval of God: On Twitter, Michael Grant and K. A.Applegate have approved of the fan reading of Marco as bisexual, with Grant believing that "all people are to one degree or another bi".
Ascended Fanon: In response to a fan discussing their reading of Marco as bi, Michael Grant has retroactively declared Marco bisexual.
Blooper: There were quite a few in early books as K.A. established a series continuity, leading to the Fan Nickname of 'KASU' (K.A. Screws Up, for the uninitiated).
Melinda Metz, writer of Roswell High (which was the inspiration for the TV show Roswell) and Fingerprints, was the ghostwriter for The Sickness and The Prophecy.
Keep Circulating the Tapes: The books have long been out-of-print, and the few fans who have managed to collect the entire series have usually only done so through sheer luck at garage sales and flea markets, or by paying a lot of money for secondhand books, paying hundreds and hundreds of dollars for incomplete sets, heavily-damaged copies. Though the entire series can be bought on Kindle or pirated in a variety of electronic formats (and endorsed by the authors themselves), those who prefer having the physical media will struggle to get the complete collection — especially the final books, which were suffering from low readership and are considered the most rare of all.
Promoted Fanboy: Erek King, the fan who won a contest to have a character named after himself. The Erek King character ended up becoming one of the most important secondary characters in the series.
Real Life Writes the Hairstyle: The book covers famously each show a main character turning into an animal. The model for Marco cut his long hair short sometime between books #5 and #10, so a minor subplot about Marco having had his hair cut was written into book #10.
Referenced by...: An instance of SCP-1468 started producing a carving of The Invasion before dying from malnutrition.
Tobias breaks the fourth wall at the end of In the Time of Dinosaurs to acknowledge that scientists dispute that several of the dinosaurs depicted in the book were alive at the end of the Cretaceous, asking the reader, "Who're you gonna believe, some scientists with nothing but bones, or a guy who was actually there?" Even aside from that, there are still some things it gets wrong, like scaly Deinonychus and a Spinosaurus that looks more like a generic "carnosaur" with a fin.
The pack mentality of wolves mentioned in the books has since been discarded by science along with the biology and mentality of many other creatures featured since they were written. For another example, dolphins are repeatedly described as calm and peace-loving animals, something that has since been contradicted by knowledge of real dolphin behavior.
In the graphic novel version of The Encounter Cassie mentions that the Alpha and Beta Wolves theory is no longer considered accurate.
Spoiled by the Cast List: The real-life narrators being listed on the audiobook covers spoils that something's unusual about #19 The Departure, #42 The Journey, and #47 The Resistence as they're the only 3 with more than one narrator listed (outside the Megamorphs, some Chronicles, and #54). The Resistance maintains some surprise as Mark Bramhall, the 2nd narrator for #47, doesn't narrate another book, leaving who his character is a mystery.
Technology Marches On: #16 The Warning, the "Internet is weird" book. It even explicitly mentions dial-up modems, which are pretty much obsolete these days.
Tribute to Fido: David Mattingly, the artist for most of the book covers, hid Easter Eggs in many inside cover pages as a tribute to his cat Orson (list of easter eggs at the link). Ironically, the one cover to include someone morphing a cat (#2, The Visitor) was done before he was hired.
Unfinished Dub: Only the first five books were released in Japan.
The original plan was for all the aliens to be the rubber forehead kind, to make a then-theoretical Live-Action Adaptation easier to produce. Scholastic told Applegate to be more creative, so she made them all Starfish Aliens instead. And then, of course, they tried to make a Live-Action Adaptation...
There were originally going to be a few more books (including one more Megamorphs which was to be released between books 53 and 54), which is why Applegate's letter at the end of the final book erroneously says there were five instead of four) that were cancelled for unknown reasons. This may also be why the number of "main series" books is 54, rather than something more regular like 55 or 60, which is truly tragic as it meant that book 54 (which would become The Beginning, the last book of the series) would have been Rachel's next narrative book had the series not been cancelled and kept on it's then-current track:note Book 55 likewise would have been Tobias' next and last narrative book, same would have been for book 56 for Cassie, book 57 for Marco, along with the final two books going back to Ax and Jake respectively, whom all missed out on extra final book covers.
Applegate has said she and Grant "planted seeds" about future plot events throughout the books, not all of which ultimately got to sprout. A major example she mentioned was the Yeerks' infiltration of the Andalite home world, which is hinted at several times early on but never fully developed.
Word of Dante: Neomorphs, a post-canon fanfic by capnnerefir, gives Tobias the surname Santorelli, from a minor canon character who, in this fic, becomes Tobias's stepfather through time-travel shenanigans. The fandom at large went with it, and it became widespread enough that on at least one occasion it was outright mistaken for his canon surname.
A twitter post from Michael Grant has stated that Marco is bisexual.
Another article confirms that Gafinilan and Mertil (identified as "a pair of Andalites") from The Other were a gay couple, something heavily hinted but not stated outright in the book itself.
Word of God: Grant confirmed in an AMA that the Rachel's final maneuver was a callback to Elfangor ramming the Blade Ship, implying that the group would survive.
Applegate and Grant based most of the characters on people they knew. Applegate has said that the character Loren is based on herself. Grant is described as a lot like Marco but it's unknown if this was intentional (probably more of an author projection thing seeing as Marco's got an Expy in Grant's series).
Applegate also said Cassie was most like her, while Marco was most like Michael Grant.
Acting for Two: Because only one Hork-Bajir costume existed, a single Hork-Bajir was the only representation of an entire race. Still a better deal than the Taxxons got; they were cut from the show altogether.
Dawson Casting: A mild example, as the age of the book characters had not yet been revealed when Ani-TV went into production.
Disowned Adaptation: K.A. Applegate was quite vocal in her distaste for Ani-TV, going so far as to insert a Take That! to it in The Return.
International Coproduction: The series was produced by Protocol Entertainment in Canada and Scholastic Productions in the United States.
Keep Circulating the Tapes: Only had 12 episodes released on VHS, and nothing else. Fans have taken to uploading the rest of the series online, even though most consider it a bad show. Qubo (one of Ion Television's spinoff cable channels) reran it in 2014.
Making Use of the Twin: When Ax morphs Jake so that Jake can be tied up while his Yeerk is starved, Shawn Ashmore's twin Aaron Ashmore appears in the one scene with two Jakes. It's also speculated that we're seeing Aaron in all the other scenes of Ax-as-Jake, but information is scarce.
It's also very obvious that there was only one Hork-Bajir costume, and the Taxxons never appear as they would clearly be impossible on the show's budget.
Only So Many Canadian Actors: Four of the main leads, as well as the minor and and supporting roles, are portrayed by Canadians since the series was filmed in Toronto.
The Other Darrin: Tom Barnett replaces Eugene Lipinski as Visser Three in Season 2. Given that this is a series where people can change into anything alive (within reason), it is more easily explained than other examples of the trope.
Recursive Adaptation: Meet the Stars of Animorphs, a companion book released shortly after Ani-TV started airing.
The games:
Creator Killer: SingleTrac was already hurting due to a string of flops after they were purchased by GT Interactive as well as many of their employees leaving to form Incognito, but the Troubled Production this game received, combined with its critical roasting and commercial failure (not helped by releasing a month before both the PS2’s North American launch and the release of Spyro: Year of the Dragon) proved to be the killing blow.
Executive Meddling: Applegate and Scholastic got caught up in the post-Columbine hysteria over violence in video games and had SingleTrac revamp Shattered Reality, which was at that point already a Troubled Production.
The graphic novels:
Foreign Premiere First: The Visitor was released in Norway over a month before it was released in English-speaking countries.
and K. A.