The Latona fountain depicts Apollo’s mother, Leto (a.k.a. "Latona" in Roman mythology).
Per Wikipedia: "When Leto was wandering the earth after giving birth to Apollo and Artemis, she attempted to drink water from a pond in Lycia. The peasants there refused to allow her to do so by stirring the mud at the bottom of the pond. Leto turned them into frogs for their inhospitality, forever doomed to swim in the murky waters of ponds and rivers."