River's stuffed Platypus.

The Stuffed Platypus is found in the "funny room" (a room which has a bunch of paper bunnies inside), as called by Tommy and Sarah. It is accessible through the door next to the stairway in the basement. The door needs to be unlocked using the key found in the book room. The platypus was given to River by Johnny after he won it on a mini-game at the carnival. River has grown attached to it over the years.
Funny room

The "Funny Room".