What is this thing?

Kinda sorta, a similar idea at least, but not the same pin configuration on the input end.

It's likely to be academic. This is most likely a semi-proprietary breakout cable for some piece of consumer gear, either a camcorder, laptop, early capture card, or similar. There were oodles of these damn things in the late 90's and early 2000's, and various manufacturers did their own things with the pin layout on the device side of the cable. There was no standard. I had several ATI capture cards that had similar ports on them, for instance, which went to ATI specific breakout boxes for composite and S-Video but would not accept a normal S-Video cable in and of themselves. Other video cards had "TV Out" ports that would connect to vendor specific breakout cables that went to composite, S-Video, and sometimes even component. And some of these while mechanically similar or even identical also were not electrically identical, just to be annoying. So you can't trust one cable to work with another device even if the plug is the same shape with the same number of pins...

Anyone interested could easily ring this out with a multimeter and figure out what the pinout is, but that may not necessarily help in determining what gadget it was originally meant to work with. If OP doesn't have whatever piece of hardware this came with, it's unlikely to be useful for anything. (And the DIN plug is not the same layout as the Sega Genesis 2 video port, either, which was my second thought.) Some pins on this may even be unused and just there for keying, or so the manufacturer could use a commodity plug, because S-Video + Composite should only need at most 6 pins even if every signal has its own separate ground.