You'll find Yawn for the first time in a locked room inside the Mansion. On an easy difficulty it took a few blasts with the shotgun to keep the giant snake at bay before he hastily retreats to fight another day.
Just remember to keep moving or you'll end up getting hit, which results in you having to switch to Rebecca to go and find a serum to heal Chris. (If you don't complete this task quickly though I think Chris may die causing you to have to start from your last save point.)
If you're going through the game quickly or trying to complete a speed run, you can easily avoid the snake, collect the item from the back of the room and escape through the door you came through, but if you're trophy hunting, you gotta attack!