The dinosaurs and other assorted saurian orders are probably quite a good model for this sort of thing, actually. Things to bear in mind: Over their their two-hundred-million-year history, the planet was largely warmer than it is today. This means more fertile regions (extending closer to the poles) and more humidity for plants to grow, hence more and larger herbivores, hence more and larger predators.
A D&D-style fantasy ecology has to share in that climate to be truly viable, IMHO.
The standard pseudo-European feel doesn't have to be absent. You just shift everything a few degrees closer to the pole, temperatures balance out again, and the only difference is that Midsummer's Day is much longer now.
The end result? An incredibly verdant world teeming with life. Grass is lusher, trees are taller, almost everything more than a couple of years old is covered in moss, and at the very least you've got giant cattle (aurochs, giant elk, that sort of thing could come in handy if you don't want to shift too far from a 'normal' feel). Insect life is a little more active; centipedes the length of your arm are not uncommon in tropical regions today, so shift that to Europe and cower in fear of what you might find if you went looking for tropical insects...
Speaking of insects, there are two choices. First, as Tonguez says, giant insects could be vertebrates with weird skin. There's another choice; insects don't have a heart or lungs, but if you provide them with a permanent magical ability to draw air through their body at great speed, it should oxygenate their cells no matter how big they get. That's how certain giant insects in my sf work, at any rate - the wind principle. OK, so there are still flaws in that (increased velocity equals lowered pressure, thus perhaps lower intake from the air), but tweak it a little and it could work. Then you have 'hurricane insects' that are intensely magical, and wizards seek them out for their Magical Parts (tm).
In general, however, there are two things high-monster fantasy should have: Lots of greenery, and lots of herd animals. Every hill should have a herd on it somewhere. Thus is my edict.