For a stationary animal in a moving fluid, air or water, the fluid pushing against the animal exerts a pressure called drag. The amount of drag depends on the size and shape of the animal.
Streamlined objects disturb the flow of the fluid less and have a lower pressure drag.
If the animal is not to be swept backward it must exert a thrust to balance the drag. If the animal wants to move forward, its thrust must be greater than the drag. When a fish undulates its body and tail, a thrust is created. The same applies to birds, bats, insects and airplanes as they fly through the air. Humans create thrust when they swim by dragging their arms and legs through the water.