I reccomend building a taming pen 6x2 with doorframes around it... And then go to tbe beaver spawns. Pick one up with you argentavis, (yes you can pick it up with an argent) and bring it back to your taming pen. Tranq it with arrows or longneck rifle and feed it some gallimimus kibble. Make sure you keep an eye on the torpor because it drains pretty fast. After that enjoy your perfectly tamed beaver.