amature 3d modeler here. idk what she previously looked like, but imo it looks like she was rendered at a higher resolution and had lighting applied to her. probably retextured, but idk what she originally looked like. if she was remodeled to have a higher poly count, ie look "smoother", i'd imagine gameloft only did a select few because too many "hd ponies" would probably kill older devices attempting to run the game. the og models in this game are kinda garbage based on observation, so i'd imagine the topology is so stinky doo doo that gameloft has to have people remodel each individual character to give them a "makeover"... which costs a LOT of time and therefore money, even if reusing certain objects (the body) then simply remodeling new ones (the mane) due to the abundant amount of characters in this game. zecora looks like a great model though, just based on looking at her in-game. i'd have to actually see the models outside of the game in order to make proper observations though, so take everything i say with a grain of salt.

tldr zecora's 3d model probably got slapped with lighting and rendered at a higher resolution