Lightning-based. So soft, it's hard to damage physically. Weak against water, so blast it with Water. Don't use lightning or it will recover HP.
The Thunder Flan is an enemy from Final Fantasy X. It is found on the Mi'ihen Highroad Oldroad and on the Mushroom Rock Road, appearing with other fiends. It will cast Thunder at a single party member in each of its turns. As with other Flans, it has extremely high physical Defense but can be easily defeated by casting a spell of the opposing element, in this case Water. Additionally, a Piercing weapon with Waterstrike will also defeat it quickly.
Thunder Flans will drop armor with Lightning Ward or Lightningproof with one additional empty slot at most. It will also drop weapons with Thunderstrike and possibly Piercing. In the INT/PAL and HD Remaster versions of the game, it will also drop weapons with the Distil Mana ability.
Thunder is the sound caused by lightning. Depending on the distance and nature of the lightning, thunder can range from a sharp, loud crack to a long, low rumble (brontide).
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.