(Covers information from the S.D. Perry novels)
The Fossil (development code: "ReH1a") was a bioweapon developed by the Umbrella Corporation in 1998. It was related to their Tyrant series.
Fossil was kept in a special stasis tank deep within the B.O.W. Envirotest A facility - otherwise known as "The Planet". As the STARS/anti-Umbrella team overran the facility, they released the Tyrant, using the distraction caused by its rampage to make their escape. Fossil proved more than up to this task, running amok through the Planet, devouring its B.O.W. inhabitants ultimately cornering and eating Jay Reston, the facility's manager.
Due to its severely intense metabolism, Fossil potentially died off following it's rampage through The Planet's facilities, as it would have inevitably starved to death after consuming all living organisms. Likewise, it might've been destroyed by Umbrella, had they attempted to reclaim or destroy the facility following its release.
Fossil had only one objective: eat. It's metabolism was exponentially increased during development, requiring the creature to eat at a vast and constant rate to maintain its own health. It stood ten feet tall and weighed roughly a thousand pounds. Its appearance was vastly different from other, more human Tyrant models; this B.O.W. and appeared as a bizarre cross between a human being and a theropod dinosaur (albeit one with no tail), hence its nickname.
To hunt its prey, Fossil had acute vision, could detect electrical charges in the air, heat signatures, and scent. The Fossil's hunting abilities were unnaturally heightened to the point it could detect living creatures through thick concrete walls with ease.
Fossil's sickly white hide was extremely thick to the point of being bulletproof, and like other models, it utilized sharp talons to kill with ease. In addition to this, its legs allowed it to move faster than previous models, ensuring it would always catch its target.
Ultimately, Fossil appears to have been a failure. While it was extremely deadly in combat, its sole driving urge (to eat everything in sight) would've vastly limited its potential as a marketable BOW. Simply put, it just wasn't smart enough to function with other units on a battlefield.