I just think it's worth pointing out that there may be a connection between one of Alola's mythicals and Hawaiian culture. I actually am not sold on the connection myself, I think Marshadow's name is likely a combination of "Mars", the Roman god of war, and "Shadow", referencing its Fighting/Ghost typing.
My interpretation was "martial" + "shadow." Of course, Mars's name is the etymology of "martial," so it's not like they're totally unrelated.
Well, if I had to guess, I say one of them will be small and cute, and the other will be big and tough.
That's my guess too. It's been this pattern since Generation III, and I'm guessing it's to cast as wide a net as possible, since fans of small and cute Pokémon tend not to like the big and tough ones too much and vice versa.
And then there's the Meltan line, which achieves both by making the basic stage small and cute and the evolved stage big and tough. Perhaps that's what this will be, actually: a single Mythical Pokémon, but one that evolves. It'll make distribution easier, that's for sure.
I think most people who are calling for a Big Ben Pokemon mean the tower, though.
That being said... I can see a Bell-based Pokemon being a lot more feasible....and quite interesting!
I know that's what they mean. The Big Ben bell is often mistaken for the tower to a similar extent that Frankenstein's monster is often just called "Frankenstein." (Incidentally, considering the original novel is from the UK, and the monster is something of a legend in the story, I think that'd make for an interesting Mythical Pokémon, though Magearna is an already-existing Mythical Pokémon based on an artificial humanoid lifeform created long ago. Or maybe a Pokémon based on Frankenstein's monster was just something on my wishlist that didn't happen. Then again, Cara Liss does seem to be something of a Dr. Frankenstein kind of character.)
We do have Bronzong in this game though, which is the closest Pokémon to a clock tower bell.