The Fowl Fountain is a fountain in the Village of Fowl Devotees shaped like an enormous crow. Water streams out of a hole in the fountain fashioned to look like a beak, and as Snicket quotes- 'the effect was hideous'. It makes it look as if the bird is vomiting.
Unknown to many, the fountain has a hidden mechanism in its eye, allowing the beak to open so at least two somewhat human-sized objects can be placed inside. Count Olaf had hid Isadora and Duncan Quagmire inside it soon after he had kidnapped them. Isadora and Duncan got out by way of a few couplets that Isadora wrote and sent to the Baudelaire children using a crow.
Its name is likely a dual reference to "foul", as "fowl" and "foul" are spoken the same.