The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
Some ideas:
Takes One to Kill One - The enemy can only be harmed or defeated by their own kind.
Became Their Own Antithesis - In the course of gaining power to defeat this enemy, the hero becomes more like the enemy. May lead to He Who Fights Monsters.
If someone already had this power (i.e. Goku was clearly always a saiyan even if he didn't advertise it to his Earth friends), that seems more like a generic The Reveal (revealing to the audience) or Internal Reveal (audience already knew; revealed to other characters).
Very common shounen trope. The protagonist is able to fight the stronger-than-human enemy because he has/gains that inhuman power. Almost always surprises(or even alarms) everyone. What's also common is that a bunch of friends with similar powers enter the show later on. Seen with Eren's titan transformation in Attack on Titan, Goku being a saiyan in Dragon Ball, Asta's demon powers in Black Clover, Ichigo's shinigami form in Bleach, and many, many other shounen series.