Metal Flan, like normal Slimes, can be found almost everywhere, but are rarer and have high defense and magic defense, making them hard to kill in the early portion of the game. When defeated they drop a lot of experience and Amethysts.
Metal Flan can be found in battles with non-metal flan (Flan, Red Flans, etc). This can result in a Metal Flan using merge into a regular Flan, which means that defeating the Flan it merged into will not give the experience that a Metal Flan would, but they will still drop Amethysts.
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.
- The Metal Flan is likely a reference to the metal slimes from Dragon Quest games that are rare to find, flee battles easily and yield a lot of EXP when felled.