there is footage of Polish Cavalry charging German tanks, only to be slaughtered moments after as if they were completely unaware as to what they were up against.

This is a myth. I recently addressed this here. There is no actual combat footage of this ever occurring, and it is an utter fabrication by the Gobbels propaganda machine. Given that the Polish Army made fair use of tankettes and actually had limited amounts of the new 7TP tank which were superior in design to the German Panzers, I find it hard to believe the Polish would be ignorant to the employment and existence of armor in 1939 :)

Poland was far from the first instance of tanks being used. Heinz Guderian in his Achtung, Panzer! actually describes the first encounter of armor on an unsuspecting force. The Germans, while shocked at these 'armored tractors' were not dumbstruck for long, and attempted to engage them with artillery firing over open-sights. Armor-piercing ammo was in great supply, SmK ammo being used to engage bunkers and fortresses long before the advent of tanks.

However, WWI artillery was generally ungainly, and to limber it in a way to fire over open-sights was time consuming. The first tank attacks of WWI were limited successes; stopped not by enemy action, but rather by a lack of concentration and a clear plan on how to use them best. Likewise, the tank's success, while certainly buoyed by their novelty, was not due to any 'panic of the unknown' on the part of the enemy, but rather the failure to adapt rapidly. It is telling indeed that the Germans for the entirety of 1916 did not feel the need to develop dedicated responses to Tank warfare.