Quite possibly the most useful animals in the game imo. It is much more than a wood gatherer. It is a mobile smithy. Anything you can do on your smithy at the base you can do on your beaver. Repair tools, craft behemoth gates, etc etc.
Castoroides are passive. If you see one check the immediate area for beaver dams. Beaver dams give you wood, cementing paste, and rare mushrooms/flowers. Beaver dams offer one of the fastest cementing paste gathering rates in the game. Once you open a beaver dam all castoroides become hostile so be quick. When you grab cementing paste from the beaver dam inventory make sure to dump all the wood or excess thing you will not collect. This is important because more cementing paste will not spawn once it's taken, a new dam needs to be created. By dumping all items the beaver dam is destroyed and a new one will be created later. Wild beavers are not worth killing. They offer little pelt if any and leave a mean bite.