It‘s not original. The joke originated in xkcd 821 (Five Minute Comics Pt. 3), however it was a little different still. The wording (besides the slur) stems from xkcd creator Randall Munroe‘s book What if?.
If you compare the image to actual xkcds, you’ll notice, that the font is different.
Confirmed. The one 'reference' book I have on my desk. Page 3. The font and line layout is completely changed. The last sentence reads: "and the wolf was like, "dude.""