First: Thanks for the video how to create monsters. But I still have some questions:
All following questions exclude the Biest Domain.
Does Monstrous Beast count as faction trait and therefore must be multiplied for higher class monsters or as a class trait and its cost stays the same regardless of the class?
If it is a class trait does it count torwards the maximum class traits?
Does it count as one of your free traits if you create a new faction? Example: A humanoid based monster must use the humanoid class creation section. Whenever you create a new class, you may take 1 class trait without additional costs, so if you want to create a monster, you always have to fill this slot with Monstrous Beast. Any further traits will cost you either additional points or progression points. Is this correct?
If I create a new monster based on an existing monster, do I have to pay progression points for the Monstrous Beast trait again although the existing one already has this trait?
Yes, Monsters can get quite expensive for point cost. Their traits they can get are very efficient and quick to really get the abilities you want. So yes Monster X abilities replace your faction traits and do multiply upwards when creating your classes. Use the Domain you start with to create your classes.
Yes it does count as a class trait if taken as a class trait. This will limit lower level monsters and keep them in check (not blowing apart the balance of lower level games).
Free Traits? The class trait a level one can take still uses the point cost of the trait but without the additional point costs you find often in class options. But yes, the Monster Trait would take up this slot. As in answer 2, this was a huge balance issue when monsters were added to playtesting. Monsters were incredible when first added to the playtesting and without a penalty like this, Monsters were as advanced as your normal class and became "super humans", having greater skill sets and more abilities. Monsters were meant to be strong (with powerful traits), but humanoids are still meant to have more the skill sets possible. So this was the reason why the trait still counts as Class Trait.
Yes, any class created with a monster must pay the progression points. This includes making a new class out of an existing one.