Henry Williams wasn’t always a believer in the Sasquatch. Growing up, he had of course heard the tales of old — of a legendary hairy, bipedal primate said to roam the forests at sundown.

First, they were pelts, then pests. But now they are emerging as something else: climate heroes. With its chisel-like teeth, a beaver can fell a tree in a matter of hours. The animals eat the bark ...