Sacktick

The Sacktick (literally a tick with a sack) is an parasite Synth enemy cut from Half-Life 2. It never went further than the concept stage and was therefore never scripted.

Depiction
Half-Life 2: Raising the Bar depicts the Sacktick as a macro virus created by the Combine. They are delivered by missile-like pods, generally used to soften an enemy position before an assault. They were originally to be thrown on the Borealis.

The Sacktick seeks out organic hosts on which it feeds and uses as food for the next generation of Sackticks, so that a very small number of Sackticks can multiply very quickly. When feeding on its victim, it makes a buzzy noise. The "food" taken from the victim is then stored into a sack that the creature carries behind itself.

The creature bears Combine markings, denoting delivery vehicle and task group.

Its delivery by missile-like pods is the only thing that made it into the final game, being used for the same purpose as the Headcrab Shell. The creature is also very similar in its insect-like aspect to classic Synth, such as the Dropship or the Gunship. It is not really known if the Sacktick is a Synth, but it is very likely.