Wendigo

The Wendigo is a large deer-like demon, wearing a bone-white mask with red eyes underneath. It is definitely one of the more adversarial enemies in the game, due to a combination of a high health pool (1000hp), high damage output, speed, and remaining invisible until receiving damage. Fortunately, Wendigo play a distinct sound upon spotting the player, and while moving around as well as leaving bloody footprints. The flashlight also flickers while they are invisible. When revealed through taking damage, a cinematic "horror" sound effect will play, and the Wendigo will be visible.

Appearance
Wendigo are legendary mythical creatures from the Algonquian peoples folklore. Typically, Wendigo are cannibalistic monsters or evil spirits, typically associated with cannibalism, murder, insatiable greed, and the cultural taboos against such behaviors. While the Wendigo are found initially in the Ruins in Episode 2, they are seen in the Escher Labs being suspended in red liquid, possibly being contained, experimented on, or perhaps even produced. Although mutilated bodies and warnings can be found before encountering the Wendigo for the first time, they are not seen infighting with the Military or the Cult without damaging each other firsthand.

Tactical Analysis
Wendigo are first encountered in the E2M2, The Ruins. The first two you fight are in an entirely pitch-black area, and afterwards are periodically found throughout Episode 2. Be sure to listen for their distinct alert sound or breathing, or watch for physics-sensitive objects being knocked around by seemingly nothing, or look out for their blood-red footprints. Spraying with a weak weapon with a high fire rate (such as the Pistol or Assault Rifle) to reveal the Wendigo, then attacking with a more powerful weapon is a very viable strategy. Be aware that the Wendigo can damage the player while remaining invisible.

Trivia

 * Several file names relating to the Wendigo refer to it as "Skinwalker" referring to their other name in media, and Algonquian mythology.
 * Wendigos are roughly analogous to Doom's Pinky Demons, as both are large but quick animal-like beings likely lacking the ability to reason, while also being invisible (at least initially) like the Spectres.
 * Mark Diaz (who works as a programmer at iD Software) suggested the addition of the Wendigo to David Szymanski.