Ignore all comments about wild castoroides being able to build giant beaver dams. They're making it up.

Giant beaver dams spawn alongside 2 Castoroides, so if you spawn in south zone 2 and see a giant beaver dam, kill all the castoroides plus some coelacanths, and you'll get another beaver dam plus 2 beavers, which again, you should kill plus some coelacanths to free up space for more giant beaver dams to spawn.

Wild giant queen bees can't build bee hives, so don't expect wild castoroides to be able to build giant beaver dams.