The Eagle Has Landed

Oneshot-- When Neil Armstrong becomes the first man to step on the moon, he finds something Unexpected with a capital U. And it's very much alive-- and equally curious.

When Apollo 11 lands on the moon, Neil Armstrong utters his famous lines as his boots land in the lunar regolith, but when he turns around he finds something no one was expecting or had ever planned for.
There's an alien on the moon. And it's blue, four-legged, and staring at him.