CM Punk
Occasionally we see MMA fighters enter the world of professional wrestling but the opposite rarely occurs. However, the UFC saw a golden opportunity when they signed wrestling star CM Punk from the WWE. They expected wrestling fans to cross over and watch Punk fight for real (via The Sportster).
Unfortunately, it was an unmitigated failure. There’s a difference between being in good shape and being an actual pro fighter. Punk lacked the natural physical attributes that an elite fighter in the UFC requires. He suffered humiliating losses to Mickey Gall and Mike Jackson before he called off his ridiculous pro MMA career.