The answer, much to my great surprise, is the octopus!
Here's how I found it.
I knew that the search terms "two legs" and "walking" would be pretty common. I can easily believe there are millions of pages with "two legs" somewhere on them, and that doing a search for [ mollusk walking two legs ] would just be hopeless.
But I figured that the people most likely to write about this kind of amazing phenomenon in the natural world are probably science writers. Thing about it: a mollusk that walks on two legs--it's not going to make the front page of the paper, but it would probably make the science page or an article in a nature magazine of some kind.
So my first (and only) query was [ bipedal walking mollusk ]
Sure enough, the first result is to a science media news report from UC Berkeley. http://berkeley.edu/news/media/releases/2005/03/24_octopus.shtml
I was also encouraged when I noticed that there were several other hits on the page that mentioned octopus in the same snippet with walking.
The details: I put bipedal first in the query because I wanted the search terms walking and mollusk to be next to each other.
Why did I do that? Is it important that walking and mollusk be adjacent and in that order?
Well.. yes! Most people don't know that word order DOES sometimes matter in web searches. In this case, I was thinking that an article (in a science magazine or a university press report) would probably say something like:
Blah blah blah walking mollusk blah blah blah...
In other words, I was trying to anticipate what I thought someone else would have written about our mysterious walking mollusk.
You can try it yourself. Try [ walking bipedal mollusk ], and the results aren't nearly as good.
By contrast, when I wanted to find a YouTube video of the two-legged walking octopus, I did NOT use the term bipedal because I don't expect YouTube descriptions to use such a technical term. Instead, I just went to Google and did a query for [ YouTube octopus walking ] and that got me just what I needed.
Key takeaway: When you're searching for something slightly obscure, try putting yourself into the mindset of someone who would be writing about the thing you're searching for. What would they write? Then, choose the key terms from that, and use those terms as your initial search.