Swans are common in zoos and can be used in ponds that are not enclosed as well, as their flight feathers are often trimmed. Right now any pond scenery we include that isn't part of a habitat is sadly barren and empty. The game currently has no ducks, geese, or swans at all, and these birds are common in zoos. I think a swan species is needed at least!
Mute swans are pretty, but I believe
trumpeter swans are bigger and are also the largest native waterfowl found in the US/Canada and are quite impressive. So I'd vote for Trumpeter swans.