Being the resourceful ones at the art of war, the Helghast created Sentry Bots to help out in some of the dirty work during their attack on the planet Vekta. While these machines had a role to play in the invasion, they were an overall failure in the war effort. The Helghast seemed to have scrapped them as they were absent from the Liberation portion of the series.
It would seem that the Helghast didn't completely give up on these bots. They have been remodeled and renamed for Killzone 2 as seen in the Killzone 2 E3 05 trailer, as well as the realtime Killzone 2 demo. Currently dubbed the A.T.A.C.(automated attack craft?), these bots have been revamped(see image below) to be faster and deadlier. The ISA have some rough times ahead.