For the masa ball: In a bowl, mix together the masa harina, manteca, horseradish, salt, baking powder, eggs and 3/4 cup water until completely combined. Refrigerate for at least 1 hour.
For the enriched broth: In a large pot, add the chicken stock, chicken legs and onions. Place over medium heat and bring to a simmer. Cook until reduced by one-quarter, about an hour. Strain the broth, discarding the onions. Remove and discard the chicken skin, then shred the meat and set aside. Season the broth with the vinegar and some salt.
For the composed soup: In a pot on medium-high heat, bring the enriched broth back to a simmer. Form the masa mixture into 8 balls and add to the broth. Cook until the centers of the balls are hot, about 5 minutes. Add the celery, carrots and reserved shredded chicken and cook until the soup is almost boiling, another 2 minutes. Mix in the cilantro and serve.