It's probably a translation problem,the cow in japanese could be called"牛"and"乳牛","牛"is a wide variety,but "乳牛"means a cow that specializes in producing milk.Of course,gnu is another animal that looks like a cattle,I know that.But this is subject to sensei's setting.However,I think japanese version is the true,"gnu" may be the wrong text,after all, Japanese is the original.