The method I used to tame was picking up the beaver with my crystal wyvern and dropping it in a taming pen. Pretty easy knockouts but their torpor drop rate is INSANITY and you can't leave then for more than 3 minutes. Feed them either mejoberries or crops (kibble is really expensive but if you want to do that, go for it).
Make sure no other Castoroides are around then bola it and shoot it with tranqs until it knocks out. Make sure you bring loads of bio toxin, because their torpor drains fast. I used this method and tamed one at level 42