
But when the battle starts, he shows up as Eryngo!

As you may know, Byakubu only appears in Power Version, and Eryngo only appears in Speed Version. And they appear in the same places in their respective versions. Looks like someone at Smilesoft forgot to change the sprite from Power Version to Speed Version. Oops. It's also funny how people always talk about the errors in Diamond and Jade, yet there are a few authentic mixups like these. Hehe.
For reference, this is what Eryngo really looks like:
