Dark Rex is a sinister being who is really half Rex/half flamebreath. He is larger than Rex in size, and has a far more feral-looking form, with torn wings, huge canines that poke through the jaws, and a single glowing red eye, and he has spikes sticking out of his back, and longer, sharper claws.
Dark Rex has all the memories of Rex, but his emotions are all darkened to an extreme, and thus he's callous to any tender moment he shared with his former friends. When it comes to cruelty, he knows no bounds. He'll gladly rip apart any victim if given the chance. He is a deceiver. He is a liar. He is a killer.
Dark Rex appears after Rex is forced to join Chomper's ranks. For the time being, he pretended to be loyal to Chomper, and tried to kill Littlefoot, only to be saved by an alimon. Because of Dark Rex's intact memory, Littlefoot became convinced Rex really had betrayed him this time. But the truth was, deep inside the hybrid's mind, the two sides, Dark Rex and Rex himself, were struggling with each other over power. Eventually Rex defeated his dark half with light, and it is assumed Dark Rex is dead for good.
Surprisingly, despite being one of the most sinister characters of Out of the Shadows, Dark Rex has never directly killed any character in Out of the Shadows.
Dark Rex's true form is revealed after Rex defeats Dark Rex. It's the form of a gigantic flamebreath. Rex and this flamebreath fought each other until Rex used the flamebreath's own fire against him, and destroyed him. Despite this, it's still not clear whether or not the flamebreath is really gone.