Junior and Karlson (1968)

A Soviet cult cartoon, so untypical for a Western viewer, especially, a little one. A boy named Malysh ("A Little One") suffers from solitude being the youngest of the three children in a Swedish family. The acute sense of solitude makes him desperately want a dog, but before he gets one, he "invents" a friend - the very Karlson who lives upon the roof. So typical for the Russian culture spirit of mischief, which is, actually, never punished, and the notion that relative welfare not necessarily means happiness made the book by Astrid Lindgren and its TV adaptations tremendously popular in the Soviet Union and nowadays Russia and vice versa - somewhat alienated to the Western reader and viewer (see User's comments below). However, both the book and the cartoon are truly universal - entertaining and funny for the children and thought-provoking and somewhat sad for grownups.

Unfortunately the movie Junior and Karlson is not yet available on HBO Max.