I recently heard someone use the expression "Now that's a whole different bag of dog food". While highly unusualy, the meaning was well understood by the audience. I know there is an actual idiom/expression used more normally than this one, but I cannot recall what it is.

"Horse of a different color" comes to mind, but I don't think that's it. Is there another equivalent expression in common use?

I recently heard someone use the expression "Now that's a whole different bag of dog food". While highly unusualy, the meaning was well understood by the audience. I know there is an actual idiom/expression used more normally than this one, but I cannot recall what it is.

"Horse of a different color" comes to mind, but I don't think that's it. Is there another equivalent expression in common use? I'd prefer one centered around the words "whole different/whole new", since I believe this expression was a play one such a phrase.