The Stuffed T-Rex is one of Lara's childhood toys that can be found in the Croft Manor of the Rise of the Tomb Raider: 20 Year Celebration. It is located in the Library behind the desk, among some of Lara's Childhood Drawings.
The T-Rex can not be picked up or manipulated in any other way, it is just decoration.
Since Lara fought against several T-Rex' in the Classic Tomb Raider games (e.g. in Tomb Raider, Tomb Raider II, Tomb Raider III), in the remake Lara Croft Tomb Raider: Anniversary, and most recently in Lara Croft: Relic Run and Lara Croft and the Temple of Osiris, this can be considered one of the many Blood Ties Easter Eggs that can be found throughout the manor.
The Stuffed T-Rex is a green cuddly toy with red claws and razor-back. It has a white belly, a friendly smile, but no hands, although its arms are rather long for a T-Rex, who is famous for having only very short and useless arms. It is about 20 to 30 cm high and sits on its rear.