Imposter

The Pokémon transforms itself into the Pokémon it's facing.

Transforms into the directly opposing Pokémon on entry.

When this Pokémon enters battle, it transforms into the opponent directly opposite it, retaining its own HP. It fails against an already transformed Pokémon, a Substitute, Illusion or a Starmobile. A Tera Raid boss with Imposter copies the host’s Pokémon.