Meaning of 狂牛 in this context? 你在說什麼？
你是又被變種狂牛攻擊喔了喔？

I'm what the second sentence means. It translates literally to something like

Have you been attacked by a mutant mad cow again?

But I'm sure that's not the actual meaning. I was thinking it might have something to do with mad cow disease, but that doesn't seem quite right either....
In the context "你是又被變種狂牛攻擊喔了喔？", it is basically just a metaphor for "are you out of your mind?", not that a mad cow really exists.