As a funny nod to "prehistoric times" when high-speed internet wasn't as common, Sebastien Gabriel, one of the game's designers, picked the T-rex.
The T-rex wasn't picked by accident either. Without internet, the Chrome Dino game was also known as "Project Bolan," after the famous singer Mark Bolan from the band "T-Rex" in the 1970s. The people who made the game also thought about adding the ability for dino game to growl or kick. The game was kept simple and "prehistoric" by not adding these features in the end.