Octocats are a trademark of GitHub. In no way do my octocats give you a right to misinterpret my feeble art to imply that I am affiliated with GitHub (other than really appreciating their services) nor to imply that I have any other right to use the octocat theme than GitHub's gracious offer to use octocats to show my ♥ for GitHub.
Unfortunately, I just found out that you are not supposed to modify the octocat.
With that disclaimer in mind, you have the license to look at the following octocats, but not to do anything else with them. They were made in a purely recreational spirit.
Under the same license, this one, too:
And this one, too:
As well as this one:
Or this one:
