What A Nightmare Charlie Brown!: Snoopy is abused and starved, becomes feral, and then dies getting pulled into a frozen lake, screaming and clawing for his life. Even if the whole special wasjust a nightmare, one could argue that this was inappropriate for the Peanuts specials' target audience.
In one strip, Charlie Brown, not having a kite to fly, attaches some string to Snoopy's collar and uses him as a kite, who uses his ears to create the lift needed to fly. But it doesn't work for long, and Snoopy plummets to the ground and SHATTERS into a hundred pieces!! Luckily it was just Snoopy having a nightmare from eating too much before sleeping, but still, that image of him shattering in front of his master must've upset a LOT of kids at the time!
In It's a Mystery Charlie Brown!, Snoopy and Woodstock sneak into the school's science fair to reclaim Woodstock's nest but also play around with some of the exhibits. When one of them gives Snoopy an electrical shock, not only does he scream, but he also briefly turns into a skeleton! Incredibly freaky for a Peanuts cartoon.
One story arc has Linus' blanket come to life and start stalking and attacking Lucy at every opportunity to the point where she stays outside all night hiding from it. At one point it lunges out of Linus' hands at her, and it has a mouth.