“I had rather be a dog and bay the moon, than such a Roman”

brutus is saying to Cassius that he would rather be a dog that howls at the moon than a Roman like Cassius. When he said a dog that howls at the moon he is saying that he would rather be a dog that howls at the moon waiting for a response and we all know that the moon can’t respond. So he’s saying he’d rather be a dog and try the impossible than be like Cassius.