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Github likely uses it because: http://unicorn.bogomips.org/

Why that project chose the name unicorn is another question though.

Yep! And here is a blog post on how Github uses Unicorns: https://github.com/blog/517-unicorn

As the above link (http://i.imgur.com/7Btc4LY.png) shows, they display a picture of a unicorn on their error pages.

I wouldn't ready any more into it than I'd read into a rabbit with pancakes on it's head (http://uwcs.co.uk/static/img/404.jpg)

Not sure, but I thought when I looked at the charts that the spikes to higher latency and exception rates looked like unicorn horns poking over the horizon.

Perhaps they are Scottish? The unicorn is a national animal of Scotland.

Well, it's on the coat of arms. I suspect if you asked the average Scot what the national animal was they'd say Golden Eagle or Stag.

Brony reference perhaps?

Doesn't look very MLP-ish in expression. I'd suspect the Invisible Pink Unicorn, except that this one's, well, visible. Perhaps a Robot Unicorn Attack reference?

Maybe it's the Unicorn instances running on their servers?