Until the development of the t-Virus, the Chimera was an impossible creation as human and fly DNA was too different to combine for bioweapons development. In their creation, it was decided that abducted women - likely homeless - would be impregnated with successfully-fertilised eggs and deliver them to produce them.
Due to their many flaws, further research into more advanced forms was never conducted, and the Chimera's basic form was never refined to the lengths of the Hunter or the Tyrant.
Several Chimeras were encountered by the S.T.A.R.S. members during the Mansion Incident. In addition, Albert Wesker, shortly after regaining consciousness after the Tyrant "killed" him, encountered a Chimera while attempting to escape, only to effortlessly kill it with a single kick due to his newfound superhuman abilities.
Still, Umbrella continued at least a limited production of Chimeras in other facilities for a number of years, as several were discovered during Chris and Jill's raid on their Caucasus Laboratory, as well as during Wesker's own raid on the facility.