Thanatos is a Tyrant modelled from the T-103. It was developed in Raccoon University by Greg Mueller, a rogue Umbrella scientist. In its first form it was unleashed onto the campus to wipe out the Umbrella team sent to retrieve it.
When you first encounter the monster, the objective is to retrieve the large sample of
T-Blood that is protruding from its back. However to do this you need to incapacitate it, and normal weapons are not powerful enough to take it down. Instead you must lure it into the
1F Passage B where there is an electric cable dangling from the ceiling. Wait until the creature is approaching the cable and activate the nearby control switch. The shock is enough to knock the monster out, allowing you to grab the sample.