Neptune are enemies from the Resident Evil series. It is a B.O.W. that was created by implanting the T-Virus into a great white shark.
It was given the development code of Fi-3 and was created specifically to test the effects of the virus on a sea creature. The creature was accompanied by two smaller offspring. During the viral outbreak in the facility, Neptune and its spawn were released from their water tanks by a scientist, who had gone mad, which flooded the entire Aqua Ring.
Two Neptunes swam in the river bordering the Raccoon University; precisely where they came from is unknown. Although these two were almost certainly killed when Raccoon City was destroyed, it is unknown if any more were in the river that managed to get outside the quarantine zone.