In frozen time:
For whatever reason, the app chose one of the more odd, roundabout routes for explaining why a horse would be in a mall pet shop. Out front, a sign appeared by the entrance reading, "Riding allowed. Horses must be accompanied at all times while inside the mall."
At the same time, the building's architecture changed, mostly getting bigger to accommodate both humans and horses. Racks for tying horses appeared, and shops changed to cater more to horse-owners.
Back at Mandy's family's shoe store, her father was helping a farmer and his teenage son pick out work boots. As he held out a pair for the boy to try, the changes rippled through them all. Racks of shoes faded away to leave more open space. Along the walls, shoe displays shrank and hardened into decorative horseshoes. Various tools and nails sat on the new workbench that replaced the register.
Meanwhile, Mandy's father grew more muscular and fit as one of the shoes in his hands became a hammer. The teenager instinctively turned his back and bent down as his body grew bigger, bursting from his clothing. Fur coated his skin as his physiology altered to that of a large horse. His foot lifted as it finished morphing into a hoof, placing itself into Mandy's father's strong hands. Mandy's father placed the remaining shoe on the hoof, and in an instant, it was a new horseshoe being expertly hammered into place.
At the horse's side, the former farmer dwindled down, losing years and gaining feminine curves. Hair lost its gray as it sprouted out long and blonde before being pulled back in a ponytail. Farming attire flowed into a female riding outfit that hugged the lithe body of the young woman now watching as her prize winning stallion got a new set of shoes.
On the other side of the shop, another farmer and his wife became a mare and her rider waiting their turn to be helped.
These and other such changes swept through the rest of the mall. A large portion of the humans present became horses with the rest becoming their owners or potential buyers.