If you want some simple gravity then check out this bouncing ball program. It's simple enough to change it into a catapult - add a horizontal movement. You can then give the object an initial upwards speed and let gravity take care of the arc. The horizontal speed will be the same all the time (just add it each time) unless you want things like wind or air friction. The main challenge after that is the collision code and creating good cow graphics (possibly rotating them mid flight ).

In case you're interested, note that Alexander Rosendal has a tutorial on a space invaders shooter w/ DelphiX here.