In early 1998, the Yawn was one of the many test subjects bred specifically as experimental B.O.W.s. The specimen escaped however, and managed to avoid recapture by the scientists in the Arklay Laboratory. It was eventually infected with the t-Virus following the leak in May 1998, and it promptly made the mansion its home, preying on any humans that happened to come across it.
The Yawn was later fought in the attic, but escaped through a fireplace before it could be killed. The Yawn would later attack one of the S.T.A.R.S. Alpha team members in the library, where it was finally killed.