I already own Minecraft (although I do have a family member who'd appreciate a gift code) and my entry technically isn't about an animal so whether or not you include me in the contest is up to you. But frankly this is creepy and awesome enough to at least deserve an honourable mention.
Ladies and GOGers, I give you... Ophiocordyceps unilateralis
Ophiocordyceps unilateralis is a parasitoidal fungus that infects ants such as Camponotus leonardi and alters their behavior. The ant falls from the tree where it normally lives, climbs on the stem of a plant, clamps its mandibles on a leaf and dies there, while the fungus consumes its tissues and grows outside it, releasing its spores. The infected ants are popularly known as zombie ants. This is a prime example of a parasitoid that alters the behavior of its host in order to ensure its own reproduction. Possessed ants march to their death and the fungus lives inside the exoskeleton.
http://en.m.wikipedia.org/wiki/Ophiocordyceps_unilateralis