The Neptune (development code: "FI-03") series of B.O.W.s were mutated sharks, created specifically to test the effects of the experimental t-Virus on sea creatures. It was named after the Roman God of the Sea to amplify the great white sharkWP's reference as the "King of the Seas".
During the viral outbreak in the Arklay Laboratory, a mother Neptune and her spawn were inadvertently released from their water tanks by a scientist, who had suddenly "gone mad" and destroyed the tanks, badly flooding the entire Aqua Ring. These sharks were later killed when the entire aquatic lab was drained, exposing them to the air. With only minimal enhancements to their bodies, they lacked any deserving mutation to survive.
Two Neptunes were encountered swimming in the Circular River bordering the Raccoon University; precisely where they came from is unknown, though these two were almost certainly killed when Raccoon City was destroyed. It is unknown if any more were in the river, or that could have managed to get outside the quarantine zone.[note 1]
The Umbrella researchers were disappointed with the creation of Neptune. The t-Virus did not significantly improve the already impressive strength of ordinary great whites, and they were still helpless out of the water.
Despite these setbacks however, Umbrella seemed to have allowed re-development of the series in the early 21st Century, with the Caucasus Laboratory having at least 3 in 2003.
↑According to the Japanese Collection for Resident Evil Outbreak, these are indeed "Neptune" B.O.W.s and not some random irregular mutation.