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Tera types to look out for!
Dragons becoming Steel. They now resist all of their weaknesses and can hit Fairy back.
Normals becoming Ghost.
Steels becoming Flying.
Electrics becoming Flying may be funny, but they can choose Ice for a devastating free BoltBeam STAB.
Bugs becoming ANYTHING: I shiver at the thought of Heracross dabbing on me with Rock Tera, Scizor's free Adaptability (Steel Tera) Bullet Punch and Bug-Tera enhanced First Impressions.
Gyarados becoming Ground: Because you just know it will happen.
Things not to worry about:
Darks becoming Ghost. They'd prefer Flying, because Ghost, at least offensively, doesn't do much for them.