Even in the event that he did give permission, doesn't seem like he thought it through that people would automatically associate it with him (or Github). I'm also not going to make assumptions about all of his talents, but he mentions nothing about illustration across any of his accounts. Did he actually make the original? Even if he commissioned it, it seems like it would be up to the artist to decide whether or not it is used for other purposes.
Just found this tweet:
Daniel Shaw @dshaw 20 Sep 11
@bkudria The angry unicorn comes from @github's old 500 page. It was originally left facing as you have it. Don't know the original source.
If I were the guys from this site, I'd really reconsider the name and the logo. Just too many crossed threads.