The Half-Life series is arguably one of the most beloved game series. It's innovations in narrative and character development (despite the main character never speaking) have cemented it as one of the most important First Person Shooters of all time.
The headcrabs are probably the most recognizable alien from Halflife. Gross little parasitic monsters that look like plucked turkey cadavers with fingery fangs. They are able to attach themselves onto humans and turn them into a sort of zombie.
I hate the venom headcrabs, not gonna draw one of those jerks.