Find a Rathalos and attack its tail to cut it off. Carve from it if desired. Continue hitting him at the head if possible until he limps, but be sure not to kill him. He might go back to Zone 5 to sleep. Go there and either throw a flash bomb to stun the Velociprey or quickly lay a pitfall trap. Waking up Rathalos can be dangerous. If you have a hammer or great sword, kick him in the head then lead him to the trap. Throw the tranquilizer bomb and you should capture the Rathalos. If the quest is not yet complete, hit him a few times, make a new trap, lay it down, lead him there, and throw a tranquilizer bomb.