Tripped-out 1991 animated short PICNIC, from the lead animator of ICE AGE

You won't find any wise-cracking sloths in James Bresnahan's 1991 short -- instead, you'll find three minutes of inspired lunacy, a barrage of wild imagery in which people vanish, black shapes take over and some excessively dark imagery is on display. Look, just watch the darn thing, though maybe wait until you get home, as it's mildly NSFW.