Warriors is a book series about cats living in the wild. They have their own social structures and medicine and religion and everything. This series was WILDLY popular during my time at elementary school. I remember my second grade teacher reading the first novel aloud to the class, brutal fight scenes and all. I loved it of course, because it was an "animal book" (tbh I still prefer stories about non-human characters).
Beyond reading the books, we would come up with our own characters. This was easy because all you had to do was decide on a cat color, rank, and two-part name. My cat of choice was black with blue eyes, and was named "Ice-Glare" (yes, with a hyphen, even though they weren't spelled like that).
"Warrior Cats" was a popular game during recess, where you would pretend to be cats (either from the book, or your own characters). Thunderclan was HUGE. Like, 90% of everyone playing the game was in thunderclan. I was in riverclan (which was like 2 people), and I remember the herd of thunderclan children running across the playground to attack us. We told them they couldn't because how can we have a battle when there's so many of them and literally 2 of us. They then walked back to their corner of the playground. This game was also banned at one point because of "exclusion". I think kids were roleplaying the story from the books, and people had to stop playing when their character died, or if there weren't any more characters for them to play as. Anyway.
I also remember taking a quiz and playing a game on the official Warriors website back in the day. I didn't understand how to play the game (The New Prophecy Quest) and my cats always starved :(