During the quest At Hell's Gate, the player will be taken to a Plasma Research Facility on Mars. Within this facility are 6 Doom-inspired plushies.
Collecting these not only gives the player an inventory plushie, but unlocks the ability to craft additional of them at an Industrial Workbench and to build them in Outposts, Player Homes, and Ships as decorations.
As soon as the player finds one of them, they will receive a miscellaneous quest objective tracking their progress towards collecting all six.
If the player misses any during the quest, they can come back later to retrieve the remaining.
While climbing the many flights of stairs into the facility, eventually the player will find a closed vent on the wall. Opening and climbing into this is where the Revenant can be found.

The Arachnatron can be found in the first room after climbing all of the stairs near the beginning of the facility on the right-hand side of the room.

The Mancubus can be found in the bathroom off of the Crew Quarters. Once in the bathroom, turn to the right and go through the plastic strip curtains.

Just before the room where the Praetor Suit is found, the player will be in a room with stairs. Going past the front of those stairs on the lower level will lead to some back rooms; the left-hand room houses the Cacodemon in an open, doorless locker.

Facing the stone container the Praetor Suit was/is in, turn to the right and follow that path to a back room. On the left will be a novice-locked supply room - inside, the Marauder will be sitting on top of a yellow container.

Across from the room the Marauder was in is a vent. Follow the short vent path to find the Slayer highlighted by a pool of red light.
