T-Rex are morphed enemies. In Ratchet & Clank: All 4 One, Ratchet could turn enemies into T-Rexes using the Critter Strike's morphing modification.
T-Rexes also appeared on display in the museum on planet Igliak.
Ratchet & Clank: Full Frontal Assault
In Ratchet & Clank: Full Frontal Assault, the T-Rex is a skin. It was available in the Predators Pack.
Ratchet & Clank (PS4)
In Ratchet & Clank (PS4), a T-Rex mask was available for purchase with gold bolts.