Wild casteroids will build beaver dams... these can be accessed as an inventory and generally have wood, cementing paste, and some sort of rare item, either flowers or mushrooms. Just only access these when no casteroids are nearby, as they will agro when you open it
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).