Wraith

The Wraith is a monster in Doom 3 and the expansion pack. Wraiths crawl on the ground and "teleport" to attack the player with scythe-like hands. It can be easily mistooken for an Imp while it's being summoned, so watch out, because it's crouched, unlike an Imp.

It doesn't really teleport, it turns invisible, becasue when it disappears, you can hear footsteps. You still cannot shoot it while it's invisible, though.

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