Lord of the Flies totally counts as horror.  Like, it's not in the genre specifically, but it doesn't have to be, it just is.  And oh my God, you've seen the animated Animal Farm too?  Gah, we need to talk more, it's like we had stuff in common as kids and got out of touch but then grew up on parallels.  Either that, or I'd ramble at you about obscure creepy animation for four hours without noticing you politely blocked me in the first three minutes.  I should introduce you to my animator friends either way, you guys would love each other.