Every Drinking Post Waterer comes with two paddles (one square and one round) to allow you to decide which style paddle works best for you
The paddles are interchangeable. Both paddles will work for all types of animals. You can try them both and decide what your preference is.
For animals that tend to wash their mouth out with every bite, you may find the square paddle allows more room under the paddle to help keep things clean.
For mouthy animals that may try to grab hold of the paddle with their teeth and remove it from the bowl, the round paddle may be best, as they will not be able to physically get under the paddle to pull up.