I don't remember exactly what causes this. It might be one of the glitches that can happen when someone else is also fighting the gym and kicks out the last pokemon at the same time as as you are trying to start a fight.
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Hitmonchan.. isn't that a past boss? did you restart your phone? Hope you didn't lose your raid pass.
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It is a placeholder. A default value that fills up the space for testing. Notice also the CP and wrong types. It happens sometimes when the game is trying to load a fight, but then is unable to finish.
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The gym turned white before you entered the battle.