T-Rex is an achievement in Halo: The Master Chief Collection. This achievement is obtained when a player find a hidden Rex sign on the level The Maw.

The sign can be found during the Warthog run near the end of the level. Normally, the player has to jump with the Warthog to cross a big gap. During this jump the player has to aim a little to the left. Done right, the Warthog lands on a platform beneath the normal platform. Here is a small tunnel with the word "Rex" written in blood on the wall. Looking for several seconds at the word unlocks this achievement.