The Aqua Flan is an enemy from Final Fantasy X, encountered in Via Purifico with Yuna. Aqua Flans use a Watera attack on each turn. Bahamut's attacks work pretty well, as does Thunder magic. It is basically a stronger version of the Water Flan.
Aqua is Latin for "water".
Flan is an open pastry or sponge cake containing a sweet or savory filling. A typical flan of this sort is round, with shortcrust pastry, usually coated with sweet syrup. It is similar to a custard tart or a South African melktert.