MUS is a MIDI variant developed by Paul Radek for the DMX sound library, which was used in 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 Accolade, Electronic Arts, and Six Pound Sledge Studio), so there is no guarantee that a MUS file will be a DMX MUS file. ZDoom only supports the DMX MUS format.
MUS files start with a four-byte signature: "MUS" then 0x1A.