Several Doom 3monsters were cut in the design process and did not make it into the finished game. Various concept designs of these monsters were shown amongst the pre-release Doom 3 promo material. One of them was a new version of the Arachnotron and Spider Mastermind (seemingly cut in favor of the other new arachnoid monsters Trite and Vagary) while the rest were apparently new designs. Eventually some of the unfinished monsters were finished and used in the Doom 3 expansion, Resurrection of Evil.
An Arachnotron concept drawing that was cut from the game.
Spider Mastermind on a wall from the video game RAGE
An early Hell Knight model.
Another early Hell Knight which looks like a Gremlin.
Early zombified Marine or Security(Possibly holding a flamethrower or a unused weapon). Taken from an unknown magazine article on Doom 3.
Early Sarge Human & Zombie form.
Early UAC Marine. This version was cut from Doom 3 but is used in the ios handheld Doom Resurrection.
Early Marine or UAC Security.
A concept art drawing of the Marine.
A high-resolution render of the different Zombie's heads.
Early version of a Mancubus.
A cut model for an unknown Demon (possibly a Guardian, Hell Knight or Pinky) resembles a headless Shambler from Quake, interesting enough it's models and textures are still located in the files.
Early Chainsaw Zombie.
Early Archvile. Later became the Vulgar in Doom 3: Resurrection of Evil.
Possible early designs of the Bruiser from Resurrection Of Evil.