1) Spiders are interesting to learn about, incredible to watch from long distances and across screens, but horrible to be around. Like the Hannibal Lecters and Heath Jokers of the animal kingdom. The different kinds have very unique hunting methods, fighting styles, movements, etc., and of course there's all those wonderful Game of Thrones family dynamics!
2) Despite being rather older than a lot of Animal-Perspective shit's target audience, I actually think I might have an answer to this. Despite the fact that a lot of it is obviously marketted toward children, and children can tell that right off the bat, the maturity of the stories is one of the main appeals. Writing/roleplaying/generally shitting on your surroundings with Warrior Cats is a lot like kids creating their own soap operas with dolls when they're un-supervised, or playing the Sims and "Exploring dialogue trees" in the bedroom and starving people to death in doorless bathrooms.
It's a harmless way to explore what you can do in writing. If you write s story about Scourge of Bloodclan, who wears a cat-tooth necklace and runs around killing other animals, it's no big deal. It's just a kid with a wild imagination. If you write a story about Pre-Genghis Mongolia in a world of constantly warring clans and one of them is headed by a guy named McKilligan who wears human teeth around his neck, you're going to be sent to the counselor's office.
That's the thing about about animals. We've seen them do literally everything on National Geographic. There's elephants getting smashed on naturally occuring coconut wine, there's graphic depictions of geese hopping on each other's goose-ginas and exchanging alien worm fluids, there's bears eating only the most pastel-red delicacies out of the crushed ribcages of deer and buffalo. Drugs, sex, and murder, is all perfectly okay for children to watch, and it gets past with a G rating, as long as animals are doing it. And we all know how kids love being edgy. In fact, when I was very, very little, before I made comics of kung-fu-fighters disembowelling each other, I... I drew Warrior Cats doing it first, and then get caught, and if the only trouble I got in for drawing the picture was because I was drawing in class and not because they were disembowling each other, I would then draw people fighting and doing the "adult approved" fatalities, and they would be completely unable to fault me for it because that's what the cats were doing. That's actually how I determined I could pass a lot of my earliest fight scenes off as "clean" in the comic books (Shitty, forced jokes surrounded by sweet but poorly drawn action) I made and stapled together, and let my friends read them, I could tell them about it and if any of them got vindictive over the immature slights we threw at each other as kids just because, they wouldn't be able to tell on me about this.
Just for fun, here was the general "rating system":
Wounds, blood drips: Okey dokey.
Large sprays/globs of blood: Only if is later found out that they were robots powered by cherry soda and/or aliens with "green", (not-colored-in) blood or "Ichor".
Wounds with lots of lines in them that look like exposed "stringy muscle" that they draw on cartoon anatomy charts: Only if the teacher doesn't know what you're doing. Try if you're feeling brave and/or lucky.
Wounds with lots of lines in them with blood around: The teacher will now know exactly what you're doing, you cheeky little shit. Don't push it.
Organs: Only okay if they're being vomitted, apparently. Intestines are slightly more okay.
Unrecognizable bits and chunks that are implied to be guts/organs: Only okay if they're being vomitted or there's so much other stuff going on in the fight scene that attention is drawn away from them.
Valentine Hearts: Only okay if used as an emote, eyeballs, or, if it must be violent, in a bloodless "Rip out your heart" stunt.
Actual Hearts: Only okay if you use them as emotes to be ironic.
People with no skin, or with large amounts of skin missing: As long as the removal happens "offscreen".
Intestines: If they just fall out and aren't being pulled, or other violence is not committed with them. A poop joke "good" enough to make the teacher smile at it is required. (Ex: "Oh poopy!" "Aw gosh, I can't control my bowels!" "Why you bein' so anal, man!?")
Loose eyeballs: As long as the optic nerve doesn't trail behind it, and there aren't any veins on it, or bleeding involved in its removal.
My friends and I played a lot of Halo when we were in 1st/2nd grade... We thought we were the edgiest motherfuckers...
3) Animals are perfectly good characters, as long as they're good characters in general.
4) In no particular order, The Chicken from Family Guy, Snowy from the Adventures of Tintin, the Luck Dragon from The Neverending Story, The Dog in I Am Legend, Animal from The Muppets. Oh, and Snoopy. Snoopy is the shit.