The popularity of the Animorphs book series eventually led to a live action television series. This was initially met with a lot of resistance within the fanbase, due to feelings that this would eventually cheapen the great book series, and that the books would translate more easily to an animated adaptation due to their truly alien aliens and the premise of morphing. But even in the midst of the backlash, Scholastic went ahead with production and the series premiered September 15, 1998 on Nickelodeon. The series didn't gain the same popularity as the books, and only lasted 26 episodes.
Bottle Episode: "Tobias," which only features Tobias and Ax, and is nothing but character material. Unfortunately, it didn't work out very well and is one of the worst episodes.
But Not Too Black: Nadia Nascimento, Cassie's actress, is considerably lighter-complexioned than the Cassie depicted in the books' cover art. Not nearly as much as it may seem from the article photo, however.
Complexity Addiction: The Yeerks only rarely use Dracon beams, presumably for budget reasons.
Or maybe because the 'Dracon beams' are just glorified flashlights.
Dawson Casting: Most glaringly for Christopher Ralph, the 22-year old actor playing thirteen-year old Tobias.
Although the characters are older than their book counterparts, as they are clearly in high school. They appear to be more along the lines of 15.
It's also worth noting that early on, the books specifically did not state the characters' ages; the show was already canceled by the time the series was finishing up, when readers discovered the Animorphs were originally 13 (and 16 by the series' end).
Particularly funny in the episode "Tobias," where Christopher Ralph has to play an even younger Tobias as the new kid in school, and faces a bully who's clearly younger and shorter than him.
Dull Surprise: Anyone who's infested with a Yeerk. A big departure from the books where Yeerk Controllers are able to make their hosts act perfectly normally, which naturally also greatly lessens the tension when it's obvious who the Controllers are.
Expy: The Ellimist becomes one of Obi Wan Kenobi. Old man, same cloak, blue and transparent.
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 painfully 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.
Power Creep, Power Seep: Jake still gets his tiger, but most of the other Animorphs have significantly weaker morphs than in the books. Rachel goes from her grizzly bear and elephant to a lion (stealing Sixth Ranger Traitor David's battle morph), Marco goes from his gorilla to a wolf, and Cassie loses her own wolf in favor of a horse.
Red Eyes, Take Warning: Of a sort. If somebody scratches at their ear in the show, it's a good sign that they're a Controller.
Unexplained Recovery: In the episode based on "The Underground," Tobias is shot with a dracon beam and falls down completely stiff, and even if he somehow survived, there's no indication that the other Animorphs took him with them in their escape. In the next episode, he's back to normal with no explanation.
Villain Decay: Visser One. In the books, she's a menacing, proactive and dangerous figure. In the show, she spends most of her onscreen time watering plants.
Villains Blend In Better: Averted. In the books, it's said that Yeerks can perfectly imitate their host's behavior so that not even those closest to them can tell the difference. In the TV series, Controllers show clear differences in behavior from their hosts. For example, Principal Chapman says that he hates a pet cat that he once loved, which makes his daughter suspicious. Ax also says that he can tell who's being Controlled by looking in the host's eyes.
Oddly enough, it's averted in the It's a Wonderful Plot episode "Not My Problem," which actually has two surprising twists about characters being Controllers. It's a good thing they weren't this effective in the real world.