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If you download the mark, it's a .zip including some smaller versions without suckers on the tentacle.

Exactly. Compare the logo in the blog post to the one in the masthead. And then go one step further and compare to the favicon (no tail there).

The small sizes don't have to convey that the tail is a tentacle, they just have to convey "GitHub".

There is a tail in the new favicon[0] It's also been the old logo[1]

Only at larger sizes. At the 16x16px version, there isn't anymore.