They can pose a particular threat in low visibility, at night (when they are active) or in caverns. While the venom is not as potent as their main competition (the cazador), it can be a problem if multiple bites are received and untreated. When they detect the player they strike with a leaping attack and a venomous bite or snap at the player without leaping. Night stalkers' original breeding ground is the Z-9 Crotalus DNA preservation lab, located in Big MT.

Packs of night stalkers can be frequently seen hunting at night east of Morning Star cavern near the nest with the radscorpion queen, south of the Broc Flower Cave, and northwest of the cap counterfeiting shack.

Night stalkers create dens in caves across the Mojave Wasteland and can be found both inside the cave or in the nearby area, behaviors they share with coyotes. They lay eggs which can be used to prepare mushroom cloud and their blood is prized as an ingredient in antivenom and hydra. A night stalker's right eye is blue with a round pupil, while the left eye is yellow with an elliptical pupil, like a rattlesnake's. They even consume food like snakes by attempting to swallow their prey whole. Their head and tail are that of a rattlesnake, including fangs and a forked tongue. Their bodies are mostly dog-like, covered in a combination of fur and scales. Night stalkers growl, howl, and whimper like regular coyotes, but hiss and rattle like snakes. Adam Figgis thought the executives in charge of X-8 could salvage the mishap in shipping and decided to let the night stalkers loose into the X-8 testing area to study their behavior. The shipping department of the X-8 research center suffered a large number of fatalities all but one employee ( Skippy) was killed by the night stalkers. The creation of night stalkers proved to be a risky gamble.

However, his memory was unclear, and he also suggested that it may have been because of a dare with Dr. Borous created nightstalkers in order to preserve the DNA of rattlesnake, which was on the verge of extinction.

Borous at Big MT, being genetic hybrids of rattlesnake and coyote DNA.

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