The only animals you can catch are things like nerf/bantha. To get these, they firstly must be on the map you are playing on. If they are, use your scout to drive to them and they flash to become yours.
However, should an enemy scout drive past them, they take ownership until you drive back again and recapture them. Once they reach your nursery, simple select your animals and then click on the nursery and they will enter it, giving you a food source.
Only the Confederacy can actually make animals out of their nursery, like those in Episode II