There's something that's been bothering me since gen 7 was released: Burn Up, Toxic Thread, and Fire Lash. All three of these attacks have extremely limited distribution to weird-ass Pokemon who aren't important in SM (although a case could be made for Ariados, since Guzma has one). The only reason I could fathom for these moves existing is if they were originally conceived for a Pokemon who hasn't been released yet. Now with Burst's reveal, I'm even more sure of it: This is the Pokemon Burn Up was designed for thematically.

Assuming it doesn't also turn out to have Fire Lash too for some reason, I think we've got two more upcoming gen 7 Pokemon who will be the "main" users of Toxic Thread and Fire Lash.