In Halo, a small creature that attempts to attach itself to the Master Chief. If the Master Chief's shields are depleted, the creature will succeed and drain health at an alarming rate (especially when multiple are attached). Having a swarm of these creatures attached to you is a sign that you are about to die. Fortunately, a single shot from any weapon will cause a creature to explode, possible fratricidally if others are nearby.
In the Metroid games, a creature about the size of a human head that attaches itself to Samus and rapidly drains energy, growing larger. Can be removed by entering morph ball mode and dropping a bomb. A good preventative measure is freezing one with an ice-based attack before it can attach itself.