HMP is a variant (and incompatible) MIDI format developed by the now-defunct Human Machine Interface Inc. company. It was used by the Sound Operating System, their middleware library for audio playback. They were the main competitor to the Miles Sound System (which used the XMI format) in the game industry in the 90s.
There are actually two different HMP formats: the original, identified by the HMIMIDIP signature at the start of the file, and a revision dated from January 31, 1995, identified with a longer signature of HMIMIDIP013195. A second revision, dated from June 15, 1995, created the HMI format.