What? I didn't write about what dragons eat?
A couple weeks ago, someone (I forget who) posted about ignoring normal biological requirements for monsters in dungeons. It reminded me that I wanted to post something -- thought, in fact, that I'd already posted something, though I can't find it -- about the biological needs of fantastic beasts.
A fantastic beast, as I'm sure I've mentioned before, is a monster like a dragon or unicorn: a fantasy creature with innate mystical or magical qualities and perhaps a hint of impossibility. This is distinct from fantasy creatures that are not actually fantastic: they are based on real-world creatures, with unusual but theoretically possible changes. An extreme example of the latter: bears don't have feathers, but there's nothing really impossible about a feathered bear. Or a porcupine cat. A dragon, on the other hand, can fly despite its enormous size and can breathe fire; despite various literary attempts to come up with theoretically realistic methods of producing fiery breath, it seems like a distinctly supernatural aspect.
I feel that it's better to wholeheartedly embrace not just the supernatural explanation for the powers of fantastic beasts, but the fantastic nature of their existence as well. These are not biological entities. They may be able to breed, where that would open up interesting possibilities (dragon eggs,) but they are not necessarily the products of breeding, and certainly not the result of evolution, perhaps not even the result of intelligent design.
Part of this attitude includes the idea that fantastic beasts do not need to sleep or eat. They may choose to do so: sleeping dragons is a strong archetypal image, after all. But dragons sleep out of boredom or laziness, hunt for the thrill of the hunt, and eat for the taste alone.
A troll in a locked room doesn't starve to death. But it gets hungry. And perhaps a little crazed.