Hacx includes an unnamed invisible enemy, which only appears a couple of times in the game, both times on MAP10. It is completely invisible and has the same ranged attack as the Thorn Thing.

The editing document included with the game makes no mention of this foe and states that the Doom object it replaces (the revenant) is unused, suggesting that its presence on the map was probably a mistake. That said, both of its appearances are in extremely prominent places and the editing doc does make at least one other confirmable mistake in its object list.

Hacx monsters
Android | Buzzer | D-man | ICE | Majong 7 | Mechamaniac | Monstruct | Phage | Roaming mine | Stealth | Terminatrix | Thorn thing | Thug | Unnamed invisible enemy