What did shakespeare mean by now i will believe that they are unicorns?

Oh, dude, so like, Shakespeare was totally being sarcastic in that line. He was basically saying, "Yeah, right, and next you'll tell me unicorns are real." It's like when your friend tells you they're going to start working out every day – you're like, "Sure, and I'm going to ride a unicorn to work tomorrow." Shakespeare was just throwing some shade, ya know?