Crazy Helen: ...It's a big roan mare. She's acting all funny. Like maybe she's been eating the loco weed. Rachel: Loco weed? Cassie: [shrugs]
The same book also has a pretty obvious reference to The Talk in one scene.
Edriss's backstory in VISSER is full of this. She spent a few years lounging around on drugs, for one thing.
It's not just Edriss's backstory either—VISSER features a cocaine-addicted actress nicknamed "Jenny Lines", Visser Three bribing a homeless man with alcohol, and various and sundry other sneaky little additions. For a series marketed at preteens, VISSER is a very adult book.
In #2 The Visitor, they get away with Marco calling someone a skank, which is a taboo even in a lot of young adult books, let alone a middle-grade one.
Human Ax's entire comedic shtick is, in a nutshell, the fact that his alien instincts make him act like The Stoner when he's in human form: he has insatiable cravings for junk food, he eats huge quantities of food in single sittings, he pronounces everything in a monotonous drawl with AcCENT Upon the Wrong SylLABle, he gets weird fixations on certain movies and TV shows, he finds random things inexplicably fascinating, etc. They never directly reference drugs, but the whole joke is painfully obvious when you reread the books as an adult.
In some versions of #22 The Solution, Rachel calls David a bastard. (This was changed to 'creep' in later editions.)
In the same vein, the words "X from hell" (roller-coaster from hell, paint job from hell) show up every few books or so. Why the censors let them by is anyone's guess.
In the third Megamorphs book, the Animorphs go to the 1930s, where Cassie is harassed by a racist. They skip right over what he says to Cassie's internal reaction, but it's made obvious that he called her the n-word.
In #4 The Message, Marco mentions having "weird dreams about that woman from Baywatch." Now, what kind of dreams might those be?
In #16 The Warning, Jake and Tobias have to morph fly to sneak onto an airplane. But only one stall is free, and they have to share. For extra fun, keep in mind that the only clothes they can morph are skintight. So, you've got two teenage boys, standing around in the same bathroom stall, all wearing skintight clothing.
There's that whole subplot in The Exposed where the Animorphs raid a crackhouse. No, really. They raid a crackhouse during a SWAT investigation. And end up tossing aside some guy who's "too stoned to move." Children's books!