It's not much of a secret, really.
All of Elliot's Beasts will receive it upon reaching a certain amount of contribution to Elliot as a Beast Master. And because Brock, Winter and Wukong had incredible contributions, they unlocked the "secret skill" first.
"Feral Judgement." Wu Kong said as he stepped forward towards the praying mantis. His bones grew and his muscles instantly bulged out. Within moments, a massive ten meter tall Monkey King with fearsome red eyes stood in his place, screaming out a roar that was totally unlike Wu Kong's calm and collected character!
RAARR!!!!
Wu Kong too a giant leap forward towards the massive praying mantis and smashed his rod down very powerfully on it!
BAMMM!!!
Although the praying mantis had very thin limbs, they were startlingly strong, and it had no problems resisting Wu Kong's frenzied smash. In fact, it didn't look like it received any damage whatsoever!
