Two characteristic that might be important for a terrestrial animal would be:
a) "Flat Legs" in order to walk straight on the ground.
b) "Lungs" instead of Gills, to breath in Air.
Here, Flat legs are not required for an aquatic animals, 'cause they float...but it is important for a terrestrial animal, and Lung is the Organ which breaths in Air in higher animals, it is necessary 'cause the original organ in aquatic animal for this purpose is "Gill" which is only applicable under the water body
Hope this helps!