Turns out that "Windsor Meat" just copied a joke from comedian Myq Kaplan, who is a vegan. You can hear him tell it himself in this Conan O'Brien set from February, 2012.
The joke begins at 4:16.
Kaplan took to his blog on Wednesday to comment on the Butcher shop stealing his joke.
I haven't read all the comments and responses, but a running theme seems to be "Vegans and vegetarians don't have a sense of humor," coming from people who think the joke was made by a meat-eater at the expense of vegans.
But I'm the vegan who made the joke. And I can take a joke, I just can't take that a joke was taken from me. (I mean, I technically CAN take it. I'm strong. From spinach. Like Popeye.)