Alright here's my current theory, working backwards from knowing the answer from OP confirming a guess without definitive evidence.

November is a mega Altaria. We know it takes neutral damage from ground and can be taunted by a prankster mon in psychic terrain because it enters the field not grounded.

How do we know Xi has prankster? Xi has prankster because we know Juliett is immune to its toxic, but not Omicron's toxic.

How can we prove Juliett isn't part poison/steel and Omicron isn't a Corrosion Salazzle? A modest, 31IV, 252EV spA Salazzle cannot deal enough damage to kill Mike (Who we know is a Victini, since it has Searing shot, which is its signature move) even if Mike has zero IVs and EVs in hp and def.

How do we know Mike isn't a Smeargle who sketched Searing shot? It was hit by night shade during the turn Juliett used protect.

Thus, November must be a mon who is vulnerable to prankster in psychic terrain before mega evolving, and gets OHKO'd by a +2 Mega Gallade using a neutral damage Earthquake after mega evolving. November is therefore an Altaria.

I think the OHKO only works on a mega altaria with 0 IV/EV in hp and def with a -def nature though.