This is the “IS-NITRO-UIC(MIDI)” development tool used to create MIDI music and audio effects on the Nintendo DS. (Released by Intelligent Systems.)

Targeted at both software Engineers and Musicians developing Nintendo DS games. This device allows the user to test Midi music tracks and audio sounds on a DS handheld in real-time.

Images Provided by @GamingLegend64 (Twitter)

As shown in the image below, developers could interface the unit to a development PC over USB for performing Firmware updates and also other software related controls of the hardware. There was also a standard DIN-5 socket connector for connecting external MIDI devices, such as MIDI Keyboards or Midi Sequencers etc.