I asked this question a few times on the stream and got myself an answer. They chose the name Packmaster, because Master Moulder sounded too much like a lord type. Sean also added that they decided to add more hero options, because of the feedback, that heroes are much more versatile than lords. That is the reason, why there is no Master Mutatior.
His Royal Highness, Phoenix King Finubar!
"It has been too long since I drew a blade in anger, Tyrion. You have been my sword, and Teclis has been my shield. But now it is time I fought my own battles!"
I used to be crazycrix, then Epic happened and I became Dubinekdubajs, which I had to change again😀