FengBoa my butt. That's Meng Huo!
Is this part of a hidden character event? I haven't seen one announced.
Actually, in showdowns, there's an item that you can find in crates called a musou spirit. Picking up one of these will allow you to take the form of any general that's in your faction at random. Since Meng Huo is in Shu, any Shu player that picks up a musou spirit has a chance of becoming Meng Huo and being able to use his moveset. However, once one side picks up a musou spirit, any other player on that side that picks it up (should the holder die and drop the spirit) will become the same general, so in the end, it all comes down to luck as to who the team will get.
Spirit orbs (the items you can get in campaigns by defeating enemy generals and winning the battle) can't be used in showdowns, so this is the only way to become a musou general in the battle mode.
Edit: changed soul orbs to musou spirits after checking the wiki.
P.S. Here's what a musou spirit usually looks like:
(from Yuri's DWO wiki)
P.S.S. Having a musou spirit usually grants you level 3-4 armor depending.