The Chimera was produced through genetic manipulation. They were created through in vitro fertilization by artificially-inseminating a human ovum with fly DNA, using the t-Virus as a bonding agent. The egg would then be placed within a woman to carry it to term.
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 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 limited production of Chimeras in other facilities for a number of years, as several were discovered during Chris and Jill's raid on their Russian HQ, as well as during Wesker's own raid on the facility.