A leash is something tied around the neck so that the one who holds it can control the one tied on it. This is the one they tie around animals, particularly dogs. A freak is an unusual person (I wouldn't say crazy, just odd/weird). Instead of a dog tied on the leash, they tied a freak on the leash in the song.

This song is a metaphor for Jon Davis' feelings toward American politics and the music industry. He feels like a prostitute to her pimp and customers abused and used. He feels "raped" by the corporate world. Like a prostitute he does all the work, yet he isn't entitled to all the rewards because some other jerk is out there to take his money.