Everyone (at least among developers) knows the GitHub's logo.

Several knows the mascot is named Octocat, it's a merge between octopuss and cat

Some knows the story behind this logo creation.

The idea for GitHub’s logo came from its “octopus merge” – the practice of combining three or more branches of development. Initially the logo’s designer Simon Oxley thought about calling his creation ‘Octopuss’ (combination of octopus and pussycat) to represent how complex code combines can create peculiar things. But in the end, GitHub decided to go with ‘Octocat’ (for obvious reasons).

The logo was created by Simon Oxley, who doesn't remember creating it, apparently.

Original GitHub Octocat designer Simon Oxley

Original GitHub Octocat designer Simon Oxley on his famous creation: "I don't remember drawing it" Simon Oxley is the sort of fellow who makes dry jokes so subtle that Americans will miss them. He's not a developer, techie, or household name in Silicon Valley.

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More information is available on Wikipedia's page of GitHub