In New Vestroia Arc 1 we see that while Brontes is a Haos Bakugan who has the ability to use Darkus abilities without an attribute change although he is still predominantly a Haos Bakugan as he only has three Darkus abilities that we know of but a larger number of Haos abilities.
I guess my question is how?
My guess would be Brontes was born sometime during the events of Bakugan Battle Brawlers when the attribute worlds of Vestroia are colliding together since we see this created Naga's gatekeepers who where also attribute hybrids with access to two attributes but favoured or had more access to one attribute more than the other.
I feel this explanation makes sense for this question since I don't think there is a canon answer but what do others think?