These are some normal headcrabs that got exposed to radioactivity and became super headhumpers, humping stronger and faster at will. Many tried to make fast headcrab cannons, but failed when the cannons exploded when the fast headcrabs humped them. When exposed to a longer time of radioactivity they will mutate and hump poison. They launch themselves at unsuspecting victims, and hump them so rapidly the victim is too shocked too move. After they are finished, they will jump onto someone's head and stick a small horn through their skull. This for some reason this causes the victim's chests to rip apart and spray blood everywhere in a spectacular 3-D explosion of awesome.