Just because a rock looks like an animal doesn't mean that there's a rat on Mars, so everybody relax.
I've seen this story popping up a few places, and it's been getting on my nerves. What you see in that picture is not some kind of Martian rat or lizard. It is a rock. It's not even a rock that looks that much like a rat or a lizard. Our brains are wired for us to recognize familiar shapes and see faces. That's all this is.