That is an beautiful octopus.
So here’s my problem. When reading that, let alone speaking that, I find the sentence clunky, awkward, and that it takes me out of the reading experience entirely. Yet, technically, the above sentence is correct. “An,” our article acting as an adjective, is still modifying the word “octopus” and not the word “beautiful.” Because “octopus” begins with a vowel sound, we must use “an,” regardless of the phonetics of the word immediately following the article.
A long angry rant follows. But there's just one problem. No one would write "an beautiful octopus." No one would tell you to write "an beautiful octopus." Because it's wrong.
It's a rare achievement to be both completely pedantic and utterly mistaken.