(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, ultimately cornering and eating Jay Reston, the facility's manager. Fossil was likely destroyed along with the rest of the Planet.
Fossil had only one objective: eat. It stood ten feet tall and weighed roughly a thousand pounds. Its appearance was vastly different from other, more human Tyrant models; this BOW looked like a bizarre cross between a human being and a dinosaur (albeit one with no tail), hence its nickname.
Fossil's hide was extremely thick, and like other models, its sharp claws 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.