MUS is a MIDI variant developed by John Romero of id Software for the Doom engine. It is a more compact version of the Standard MIDI File format, almost identical and easily converted to MIDI. Since it is much more limited and offers less features, conversions the other way around, from MIDI to MUS, are not advised.
Note that at least three other MUS formats were developed (by, , and Six Pound Sledge Studio), so there is no guarantee that a MUS file will be a Doom MUS file. ZDoom only supports the MUS format developed by Id Software.
MUS files start with a four-byte signature: "MUS" then 0x1A.