I earned the Silver Panzer Hydra Medal from the 'defeat the Panzer Hydra 10 times' achievement, but when I used it the item was consumed and I didn't unlock a challenger. :( I assume this is because the other challengers aren't available in the throwback period, but it's a bit of a bummer to lose a million sP item for nothing.

Just to clarify: did you get the message that you unlocked a hidden opponent or did nothing happen at all?

explained over here that the opponent should be unlocked, it just won’t show up in your available challengers until Masquerade: Silver Panzer Hydra Medal?

A new page opened as it usually would when using a challenger unlock item, but it was blank. I should have taken a screenshot. I guess I'll wait and see if the seven headed hydra is available during Masquerade and open a new thread if it didn't work properly. :)

Thanks for clarifying! It makes sense that it would be blank since the next challenger isn't active. I was looking through old threads and found this one Panzer Hydra challengers are not unlockable. where it says you would normally receive the error message "You cannot unlock the Panzer Hydra outside of the Masquerade Holiday!" if you tried to use it in the off season. But we're kind of in uncharted territory now. I really hope it did unlock for you and that the challenger shows up properly once Masquerade rolls around!