This sounds like a coyote, fox, panther or bobcat. Swans usually stay in the water at night. But, they can also go toward the bank if they want to stretch, preen or eat. Just after dusk until an hour after dawn is when predators come out to hunt. Usually, a large animal such as a coyote or bobcat will go for the head and strip the neck.

Captive swans or other waterfowl that cannot fly should be placed in a secure pen (one that cannot be climbed into or dug under) at night to keep them from predation. The Regal Swan