You all know what a bestiary is, right? If you don't, here is the definition kindly provided by dictionary.com:
bes·ti·ar·y [bes-chee-er-ee, bees-]
noun, plural -ar·ies.
a collection of moralized fables, especially as written in the MiddleAges, about actual or mythical animals.
A bestiary really is a fantastic source of inspiration for the fantasy writer. I suppose it could be inspirational for the fiction writer as well, though they'd have to do something really inventive to make a cockatrice compatible with real life. However, the article I read kind of gave examples of how to do that, such as making the monster something inside you, an insidious dark aspect of your character that the protagonist has to defeat in order to grow and conquer.
One of the best things about the bestiary is that it is full of real and fantastical beasts. You can take those you like and adapt them to your needs. You don't have to stick to one set diagram. You can use what the bestiary gives and perhaps put a little twist on the creature's character. Why does the manticore sup on human flesh? Why can't you cut off one of the hydra's heads without causing two more to grow in its place? What happens if you cross a centaur with a gryphon? Kind of interesting, isn't it? I thought so.
You can access the Children's Writer article here. It was really fantastic. While you're at it, why not take a trip through the whole newsletter? It's a brilliant newsletter, full of tips, insight, and inspiration... sort of like a bestiary. :)