Popup

From Guild Wars Wiki

Within the context of Guild Wars, popup refers to a foe or group of foes which are initially hidden, only appearing after a trigger condition (generally coming within a certain range) is met. Foes which are commonly popups include Nightmares, Mantid, Torment Claws, Oni, Mandragor, Am Fah, and Jade Brotherhood. Many of these can often be seen beforehand, by holding down the CTRL key while moving. The popup's name will usually appear, very briefly, the first time they come into sight range, allowing you some advance notice of the impending ambush.