You know, I can understand why the troops on Tatooine wanted to jump onto a Dewback. After all, it's a big, tough mount, and probably intimidates the heck out of passers-by.
But why some Clone Trooper would jump on a Slug, I have no idea. Unless it was like that racing snail from "The Neverending Story" it would be a lot slower than you could walk. Plus, it's probably really gooey.