solidmon is a modification for Quake by Jeff Epler. This modification makes it so that all Monsters have solid bodies (Ranger does not as the author played Singleplayer more than Deathmatch), which can be shot at to Gib them. This was designed in response to complaints about the Monsters having "ghost-like" bodies that couldn't be interacted with.
This modification was updated by Jason Carter, responsible for the bounding box size for Monsters, in GIBBIN3.
Note that this particular mod was packed in a .PAT file which deletes Weapons.qc upon compiling, making it impossible to run the QBSP without ignoring the changes that were supposed to be made to that file or adding them manually.