I heard a story about a devotee being told to feed an animal. Who was told to feed ants and why?

The story you are likely thinking of involves the devotee Kashiram. While the provided text doesn't mention feeding a black dog, Chapter 0 does recount that Kashiram had been advised by a different saint named Janakidas Baba to feed sugar to ants. This advice was so important to him that he always carried a bundle of powdered sugar for this purpose. This act of charity was so dear to his heart that he was willing to risk his life to protect the sugar bundle from robbers during an attack, showing the deep value he placed on the saint's words.