Bhut

The Bhut (wut-wut) was one of David "Zeb" Cook's bright ideas for X4-5-10: Desert Nomads series, which (somehow) didn't make the Companion Set as the malfera et al. would. Look for them instead in AC9: Creature Catalog and various Mystara supplements.

In daylight they take the form of humans (and can't be sussed out by Expert-level know-alignment!). At night they own their morally-true form: mostly-reptilian, with shaggy hair and claws and fangs (hey, Triassic Park!). Then "they hunt humans and demi-humans for food" (per AC9), in bands of 2-8. Home base is best sited somewhere most humans would be transient... like, oh, beside a mountain pass where Iranian-themed merchants and army camp-followers are going to-and-fro.

They shun most weapons, even in human form, so the monk is a popular cover in-between predation. Instead bhuts use their bite which is numbingly cold, demanding a Paralysis saving-throw. Losing that 'throw costs -2 to-hit rolls and a loss of Initiative for 1-4 rounds.

They share undead immunities: poison, for a start. Where they must take saving-throws, those are at "a level higher than their Hit Dice" usually "7+2**" in Basic Dungeons & Dragons terms.

Here's where Zeb's design got stupid, likely costing this monster its Companion berth: a single hit from a blessed weapon will kill one instantly" That; and its name, glorious to ten year old gamers for a generation.

The bhut's origin is from the Indo-Aryan bestiary. Scott Greene has a properly-undead "bhuta" in Tome of Horrors, but that's more like a revenant; not the same thing. Even Greene didn't want Zeb's version so that should tell you something.