Wraith/Doom 3

The Wraith is a monster in Doom 3 and the expansion pack. Wraiths crawl on the ground, can teleport and attack the player with scythe-like hands. It can be easily mistaken for an Imp while it's being summoned. Unlike an Imp, a Wraith can crouch.

Tactics
Wraiths cannot be killed while invisible, but during reappearing and disappearing animations they can still be damaged.

Trivia
In the Making of Doom 3 book, Id wanted the Wraith to form out of a cloud of insects, to hide in bodies and such, but it was scrapped since the player wouldn't even be able to tell what the cloud was in the dark environment.