Back then they have believe that the Lion is the king of all beasts because of its strength, agility, and able to flex in many positions and has sharp claws and long sharp teeth and which an T-Rex is limited. Most animals fear them because of those reasons which they know it isn't even a challenge. And so they had use a man head and a lions body to put fear in someones eyes, like a scare-crow. On this image they used the head of an lion and the body of something else. If a T-Rex trip and fall during a battle with a lion, it will be defenseless against the lion.
Long-necked lions appear to have been a popular motif in Egypt. Yes? You agree?