For the creature from George Romero's cancelled film, see Yawn (Romero).
The Yawn (ヨーン,yōn?) was an experimental B.O.W. test subject created at the Arklay Laboratory. It was nicknamed "Yawn" in reference to the way its mouth opens before attacking, which resembled a yawn.[excerpt 1]
Adders lived around the Arklay Mountains, with a number residing in the trees in the Spencer Mansion's courtyard. One of these snakes was taken inside as a research specimen and infected with an unidentified strain of t-Virus. The Yawn grew to a considerable size and, hormonally unstable, became too aggressive to control. As the Epsilon strain spread out across the laboratory personnel, they were no longer able to contain the snake and it went on a rampage and likely killed a number of researchers while making the entire mansion its territory.
The Yawn encountered S.T.A.R.S. Bravo team members, Richard Aiken, and Rebecca Chambers, as they explored the mansion, hunting them through individual rooms until it cornered them. Aiken saved Chambers from harm, but was mauled in the process and exposed to its venom.
The Yawn was later fought by a member of S.T.A.R.S. Alpha Team 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 again in the library, where it was finally killed.