(Little Osomatsu) was a comic created by Japanese cartoonist Fujio Akatsuka, which ran in Shonen Sunday magazine from 1962 to 1967. There were various revivals and remakes of the comics from the 1970s to all the way up to the early 1990s.
The comic was a hit and gave Fujio Akatsuka the mark as a cartoonist, later creating many more titles. You can read English translation of the very first story here
The basic premise is that it's about a misadventures of a family with identical sextuplets. One of the running gags in the early story was that noone can tell which is which because all six looked exactly alike (clothing, hairstyle, etc.) This was later dropped as the comic went on.
However, Fujio went into a rut because the characters didn't have much personality, aside from them being sextuplets. He later introduced side characters Chibita, a tiny bald kid that loved to play pranks on them, and Iyami, whose designs can be described as a Japanese parody of American stereotype of the Japanese (does that make sense), complete with squinty eyes and buck teeth. He like to present himself as being from France even though its hinted that he never even went there.
The two characters were a hit with fans, though, mainly because they were actually interesting. As time went by more and more stories were dedicated to them, to the point that they took over the series. Osomatsu and his siblings barely appeared and eventually were relegated to being side characters. At this point they might as well should have changed the name of the comic to Iyami and Chibita.
This trait went into the two animated series, a black and white one from 1966-67 and a color revival from 1988-89.
The 1960s series were more faithful to the comic and many episodes did at least try to feature Osomatsu and his siblings prominently.
This series was thought to be lost for years. It wasn't until the early 1990s when 16mm prints of every episode were found in a TV station warehouse. The cartoons are an interesting oddity and many are even funny, but the poorly done animation makes it hard to watch sometimes. It was done at Studio Zero, which at the time was one of the most shoddy animation studios in Japan.
Here's one with Osomatsu being prominent (until Iyami shows up half-way through)
Unlike most Japanese cartoons Osomatsu Kun didn't take strict character setting seriously. Many stories were stand-alone and in some cases there were a few where Chibita and Osomatsu would meet for the first time, despite interacting alot in other stories. Sometimes they would do things like making Chibita a businessman, a political leader, or a former safecracker who got released from prison after many years. It didn't matter to Fujio that Chibita is a kid, if he thinks of something interesting with the character, he'll do it even if its impossible.
Here's another black and white episode illustrating the above statement. This one is better-done than the others from the series although it's still shoddy in places.
In the color revival the lack of focus on Osomatsu was very evident. They barely made an appearance in the THEME SONG, the studio opting to focus it entirely on Iyami instead.
The 1980s episodes were wackier than the 1960s show. While it had its problems the animation was better done here. Here's one of the episodes:
EDIT: I'd figure that I'd post a page from the (later) comic. Not too many online, but I found this, the original comic version of one of the black and white episodes I put up.