Tualapi

From Srafopedia

Gigantic, white malevolent birds that periodically attack Mulefa settlements, destroying buildings and raiding food stores. Those beings are, unlike the Mulefa, not intelligent. Father Gomez gains control of the Tualapi by killing one of them with his rifle.

Their wings are placed fore-and-aft, and two characters confuse them with sailing boats from a distance. They have powerful legs that give them the ability to swim very quickly, though they are ungainly on land. They appear to be flightless.