Most people think it's mostly anime mashup music videos, but there are others that were mashed up as well, such as TVCMs, reality shows, movies, video games, other viral videos, etc.
Several people think the similar YTPMVs came first, but according to Know Your Meme...
"The earliest form of MAD culture can be traced back to personal exchanges of MAD tapes--cassette tapes containing re-edited, mash-up footage of various newsclips--among Japanese university students in the 1980s. According to this MAD FAQ page, a couple of alumni students from Osaka University of Arts & Music are cited as the original pioneers of audio MADs as well as anime MADS. The term "MAD" stems from one of the early cassette tapes originally titled "Kichigai Tape" (Tape of Madness), but for some reason, it became more commonly known by the English equivalent."
Even between both, there was an episode of Muppet Babies where the cast entered a music contest with a tape that was cut and taped back up in scrambled parts. That was the late 80s. And there was this rare home video sent to the original Bob Saget AFV where a guy kept changing channels that all showed only paid program TV ads in a hilarious mashup. And that was the early 90s.
Here are two examples. You can share your favorites here, too.
Huh, interesting. I've seen Anime MAD videos on Youtube and NicoNico and I thought they're mainly for anime.