In the early 1930s, the Betty Boop cartoons were among the most surrealistic and racy you could find. If pre-code animation had a picture next to it in the encyclopedia, then the most appropriate one would be of Betty herself. Though not the greatest Betty Boop cartoon, The Old Man of the Mountain (1933) is one of the greats, with fluid animation, trippy gags, and Cab Calloway scat-singing over the whole thing.
The cartoon is full of adult subject matter. The song Calloway sings over the action references drug use and the Old Man's plans for Betty are explicitly sexual, with him coming close to tearing off her clothes at one point. There's even a bit of implied rape when Betty meets one of the Old Man's former victims going down the mountain, a female hippo with three babies with whiskers in tow! It just goes to show, animation was not always stuck in the infamous "age ghetto." This stuff was for unashamedly adult viewing and unlike stuff like Family Guy, it didn't need to be infantile to entertain.